vin PHYLUM MOLLUSCOIDA 331 



mouth, and a circlet of ciliated larval tentacles; the anus is 

 situated on a prominent process. 



The ectoderm of the process on which the anus is situated 

 subsequently becomes involuted to form a deep pit (C, iv}. The 

 metamorphosis from this point is completed with great rapidity. 

 The pit at the side of the anal elevation becomes everted (D), 

 and the alimentary canal of the larva is drawn into it (E}, the pro- 

 jection thus formed, which grows out at right angles with the long 

 axis of the larva, becoming the body of the future animal ; the 

 tentacles are drawn into the stomodaeum and there become broken 

 up to form the lophophore of the adult : the hood-like pne-oral 

 lobe is taken into the stomodasum and there digested. 



CLASS III. BRACHIOPODA. 



The Brachiopoda are the fabricators of the well-known " Lamp- 

 shells " found in most parts of the world. They occur in the sea 

 at various depths, and were formerly classed under Mollusca, their 

 characteristic bivalved shell being compared with that of oysters, 

 mussels, &c. 



1. EXAMPLE OF THE CLASS Magellania (Waldhciinid) lenticularis 



or M. flavescens. 



Magellania lenticularis is found in great numbers, at moderate 

 depths, off the coast of New Zealand. An allied species, M. flavescens, 

 is equally common in the Australian seas, and several other species 

 are known in various parts of the world. 



The body is entirely covered by a shell (Fig. 265) of oval form, 

 and pink colour, composed of two pieces or valves, one of which, dis- 

 tinguished as the ventral valve (v. v), projects beyond the other 

 or dorsal valve (d. v), in the form of a short conical beak (b) perfor- 

 ated at the end by an aperture, the foramen (b), through which 

 passes a dark brown stalk or peduncle (Fig. 266, B, pd} of horny 

 consistency. In the natural state the peduncle is attached to a 

 rock or other support, and the animal lies with the ventral valve 

 uppermost and with the valves gaping slightly. The pointed or 

 peduncular end of the shell is considered to be posterior in posi- 

 tion, the opposite end or gape anterior. 



It will be convenient to consider the shell first. Both valves are 

 deeply concavo-convex, of a pinkish colour outside, white within. 

 The ventral valve (Fig. 265), as already stated, is produced poste- 

 riorly into a beak (b), terminating in a foramen (/) for the peduncle. 

 The distal margin of the foramen is left incomplete by the shell 

 proper, but is closed by a small double plate, the deltidium (d). 

 Immediately anterior to the beak is the curved hinge-line along 

 which the valve articulates with its fellow, and just anterior to 



