278 



THE LAMELLIBRANCHIA 



which extends from the anterior adductor muscle to the point 

 of separation of the two siphons, and surrounds and is continuous 

 with the foot. This septum, therefore, has exactly the situation 

 and the relations of the branchial septum of the majority of the 

 Lamellibranchia, which divides the pallial cavity into two chambers. 

 The group is derivable, more or less directly, from the Anatinacea, 

 through the series Lyonsiti, Lyonsiella, Poromya, CetoconcJia, Cuspidaria, 

 in which one may observe a gradual increase in the amount of 

 muscular fibre in the gill filaments or in their reduced equivalents, 

 so that any objection to the branchial origin of the septum, because 

 of its muscularity, cannot hold good. The muscular septum is 

 inserted on the shell, especially in the neighbourhood of the two 

 adductor muscles. The origin of the anterior and posterior exten- 

 sions of the septum and of its muscular attachments to the two 

 extremities of the shell is to be found in the physiological contrac- 

 tions necessary to create a current of water on the respiratory 

 surface of tl^e supra-septal chamber. The septum is, in fact, always 

 pierced by paired orifices, which admit of the passage of water. 



The Septibranchia are all marine, 

 inhabit considerable depths of the 

 sea, and are carnivorous. The order 

 only comprises one sub-order, the 

 Poromvacea. 



CLO 



FAMILY 1. POROMYIDAE, Ball. 

 Siphons short and separate ; the bran- 

 chial siphon provided with a large 

 valve. Foot pointed and not byssi- 

 ferous. The branchial septum bears two 

 groups of transversely elongate orifices 

 on either side ; these are formed by 

 a few branchial filaments, with or with- 

 out junctions. The palps are large. 

 All the members of the family are her- 

 maphrodite. Genera Poromya, Forbes ; 

 no pallial sinus ; British (Fig. 249). 

 Dermatomya, Dall ; a pallial sinus 

 present LiopistJia, Meek ; fossil from 

 the Cretaceous. FAMILY 2. CETOCON- 

 CHIDAE, Ridewood. Branchial septum 

 bearing three groups of orifices on each 

 side ; these orifices are separated by 

 rudimentary branchial filaments. Palps 

 large ; siphons short, separate, the branchial siphon with a valve. Genus 

 Cetoconcha, Dall ( = Silenia, Smith); abyssal (Fig. 250). FAMILY 3. 

 CUSPIDABIIDAE, Fischer. Siphons long and united, their extremities 

 surrounded by tentacles. Foot narrow, with a rudimentary byssus. 

 Palps greatly reduced or absent. Branchial septum pierced by four or 



m o 



FIG. 250. 



A ventral view of Cetoconcha, removed 

 from its shell, a.o, anterior septal 

 orifices ; a.p, anterior palp ; /, foot ; m, 

 mouth ; m.o, median septal orifices ; pa, 

 mantle ; p.o, posterior septal orifices ; 

 p.p, posterior palp ; se, branchial sep- 

 tum ; si, retractile branchial siphon. 

 (After Ridewood.) 



