These things grow in sediment, and on, or in, rocks across a wide range of tropical shallows. Most of the species from long tubes, with a bulging, porous plate and a frill at one end. Looking closely, you may a pair of tiny oval bumps on the side, up near the frill. In the photos here, they're much more obvious, but you can't see the details of the plate so easily.
They have those tiny bumps because they're bivalves.
They're also quite large, over 30 centimetres long, and very odd to handle - sadly, the ones that were donated to the Western Australian Museum whilst I was there were almost useless because the collector hadn't recorded where she found them.
This page - THE GEOLOGICAL SOCIETY OF WASHINGTON 1158TH MEETING - has this to say about a Cretaceous relative
"Ascaulocardium armatum (Late Cretaceous) - the ultimate variation on the bivalve paradigm." Pohayda says a paradigm is fancy word meaning "example," and Ascaulocardium is a pretty fancy clam, albeit a weird one. Some commercial fossil dealers collected this thing that looked like twisted calcified spaghetti from Coon Creek, Tennessee, along with gobs of Cretaceous mollusks, for which the locality is famous. After elaborately preparing this odd jumble of bent plumbing made of calcite, they brought it to Norm Sohl to find out what they had.
It turned out to be an uncanny clam, well outside the ordinary range of pelecypod shell architecture, which Pohayda and Sohl dubbed Ascaulocardium armatum, meaning "the bag-piper's heart." Its living relatives, the clavagellid clams, live in the clear, shallow seas of subtropical to tropical shelf areas, whereas the bag-piper's heart is known only from Upper Cretaceous rocks of the Gulf and Atlantic coastal plains, and probably lived in cooler water.
Living clavagellid clams are popularly known as the "watering pot," or "salt and pepper shaker" shells. That's because at the end of a calcareous tube there is a bulbous calcareous plate, perforated by a series of pores, or tubules, more or less similar to the spout of a gardener's watering can. The whole affair looks remarkably like a marital aid that not even a Parisian purveyor of paraphernalia promising passionate pastimes would dare to display publicly. So much so, in fact, that one species was named Brechites penis by none other that Jean Pierre Baptiste Lamarck, that risque French evolutionist of the 18th century who promoted the inheritance of acquired characteristics. As if to show he was no sexist, Lamarck married four times, and named a second clavagellid clam Brechites vaginiferous! By whatever name, these thoroughly modern clavagellid clams burrow, living vertically in a tube with the spout end down. Burrowing is rapid and efficient, achieved by pumping water down and out the spout, with complete burial attained in only a few seconds.
What has this to do with its Cretaceous relative, the bag-piper's heart? Juan Pohayda told us the four long tubes at the front end, the triple junction of tubes coiled around the central crypt, and the crown of tubes at the back end were all structures for hydraulic burrowing and deposit feeding. So if the paradigm for this pelecypod stirred your prurient interests, remember, clavagellid clams really carried inside plumbing to an artform, and back outside again, information only GSW and PSW are privy to.
I'm not sure how seriously you can take that report tho, since it was Linnaeus that described them ( picture above is from the Linnaeus 300 series at Raffles Museum), and they spelt molluscs wrong.
How a bivalved clam can end up looking like that is somewhat puzzling in and of itself , altho the nice people here did a study to investigate it
The bizarre watering pot shells of the clavagellid bivalve Brechites comprise a calcareous tube encrusted frequently with sand grains and other debris, the anterior end of which terminates in a convex perforated plate (the ‘watering pot’). It has not proved easy to understand how such extreme morphologies are produced. Previously published models have proposed that the tube and ‘watering pot’ are formed separately, outside the periostracum, and fuse later. Here we present the results of a detailed study of the structure and repair of the tubes of Brechites vaginiferus which suggest that these models are not correct. Critical observations include the fact that the external surface of the tube and ‘watering pot’ are covered by a thin organic film, on to the inner surface of which the highly organized aragonite crystals are secreted. There is no evidence of a suture between the tube and the ‘watering pot’ or that the periostracum of the juvenile shell passes through the wall of the tube. Live individuals of B. vaginiferus are able to repair substantial holes in the tube or ‘watering pot’ by laying down a new organic film followed by subsequent calcareous layers. Brechites vaginiferus displays Type C mantle fusion, with the result that the whole animal is encased by a continuous ring of mantle and periostracum, thereby making it possible to secrete a continuous ‘ring’ of shell material. On the basis of these observations we suggest that watering pot shells are not extra-periostracal but are the product of simple modification of ‘normal’ shell-secreting mechanisms.
Not much else is known about Clavigellids, sadly - not even their life-cycle is understood with any confidence. But if you're interested in one of your very own, Conchology Inc. has some for sale, here