384 LOWER FORMS OF MARINE LIFE 



EphippddoiCia macdowgcdli, which invariably takes up its abode in the burrow of 

 a species of prawn (Axitts plectorhynchus). When the rock is soft, the prawn 

 drives its burrow straight, but if the ground be hard the burrow is driven into a 

 small mound of sand previously constructed with much labour by the burrower. In 

 either case the hole is lined with soil on which grows an orange-coloured sponge, 

 while on both sponge and soil live the molluscs, which take up little room owing 

 to the flatness of their valves. In these burrows are two species of Ephippodonta, 

 and certain other invertebrates which have not hitherto been found elsewhere ; 

 their presence being due, perhaps, to the supply of seaweed stored by the prawn in 

 its burrow as a provision against bad times. 



Cockles, Cardiidce, as typified by the well-known Ga/rdium edule, are repre- 

 sented by more than a couple of hundred species, collectively world-wide in 

 their distribution ; on the other hand, their near allies, the clams (Tridacna), 

 are restricted to the Indian Ocean, the China Seas, and the Pacific. The shells of 

 these clams are the largest among all bivalves, sometimes measuring a couple of 

 feet or more across, and massive in proportion, with strong vertical ribs. Those of 

 the commonest species, T. gigas, are often used as basins for both sacred and 

 domestic purposes ; and it was always on one of these shells that the ancient 

 Greeks represented Aphrodite rising from the sea. Some pairs of valves exceed 

 500 lb. in weight, to which another 20 lb. must be added for the soft-parts. Giant 

 clams grow in large number in the still central lagoons of coral-islands in the 

 Indo-Pacific, and are noticeable for the vivid colouring of their soft-parts, which 

 present a splendid iridescent blaze of blue, violet, and yellow ornamented with 

 various bizarre patterns. 



Reference has already been made to the shells of the genus Trigonia, note- 

 worthy on account of the beautiful ribbing of the pale purple outer surface, the 

 nacreous interior, and the complicated ' teeth ' forming the hinge. These belong to 

 a very ancient family, the Trigoniidce, and in the Australian seas represent a group 

 which abounded in the Oolitic rocks of Europe, where the remains of relatives of 

 the modern Port Jackson shark are likewise found. 



Presenting a certain superficial resemblance to bivalve molluscs, 

 Lamp-Shells. . . 



the lamp-shells or brachiopods are so essentially different in their 



organisation that they constitute a class, the Brachiopoda, by themselves. 



Although, like bivalve molluscs, they have a double shell, often united by a hinge, 



its two constituent valves, in place of being right and left, as in bivalves, are 



respectively back and front, the front or ventral one being usually larger than the 



other, and furnished with a perforated beak, this giving rise to the name lamp-shell, 



from a fancied resemblance of this valve to an ancient Roman lamp. In certain 



species (Lingulaj the shell is horny and tongue-like, but it is more frequently 



composed of calcareous spicules. Some lamp-shells are provided with a hinge and 



an internal calcareous skeleton, but others lack both these structures. Most of 



them have a pedicle or stalk by which the shell is attached to a rock or other 



object, this stalk protruding through the perforation in the larger- valve, when 



such perforation is present. A common species, which has a vertical range of from 



5 to nearly 1500 fathoms, is the widely distributed Terebratula vitrea, easily 



recognisable by its whitish translucent ovate shell, of about an inch in length. 



