TTPE M0LLU8GA. 339 



and the reproductive ducts communicate with, the proximal 

 portion of the nephridia. To this order, which represents the 

 most primitive Pelecypods, belong the genera Nucula (Fig. 

 151), Yoldia, and others. 



3. Order Filibranchia. 



In this group the gill-filaments have elongated consider- 

 ably and commenced to bend upwards at their ends to form 

 the outer and inner lamellae {Anomia; Mytilus, Modiolaria, the 

 mussels ; Area). 



3. Order Pseudolamellibranchia. 



In this the gill-filaments show a tendency to become 

 united together and the inner and outer lamellae are united 

 {Pecten, the Scallop; Ostrea, the Oyster). 



4. Order Enlamellibrauchia. 



In which the gill-filaments are united to form continuous 

 lamellae. To this order belong the majority of forms, such 

 as the fresh-water mussels Unio and Anodon, the small fresh- 

 water Cyclas, the hard-shell clam or Quahog Venus, the soft- 

 shell clam Mya, the razor-shell JEnsatella, the boring-shell 

 Pholas, the ship- worm Teredo, and a very large number of 

 other genera. 



5. Order Septibranchia. 



A small group in which the gills are reduced to a muscu- 

 lar perforated septum dividing the suprabranchial chamber 

 from the more ventral mantle-chamber (Cuspidaria). 



Development and Affinities of the Pelecypoda. — The larva which is 

 characteristic of the Pelecypods resembles a Trochophore very closely in- 

 deed and may be described as a Trochophore provided with a bivalved 

 shell- In certain forms the characteristic ciliary bancfe may, however, 

 be very much reduced, and in the fresh- water mussels {Unio, Anodon) 

 a remarkable secondnry larva known as the Glocliidium is developed. 

 The ova undergo their development in the interlamellar spaces of the gill 

 plates, and the shell-valves assume a somewhat triangular shape, the apex 

 usually constituting a somewhat curved tooth, while smaller teeth may 

 also be present on the edges. Each mantle-lobe is provided with four 

 tactile papillsB on each side, the slightest stimulation of which causes the 



