PROTOBRANCHIATA FILIBRANCHIATA. 345 



laria marmorata in the test of Ascidians ; Vulsella in sponges ; 

 Montacuta on Spatangids ; Entovalva is parasitic in the oesophagus of 

 Synapta. Some species are very active, leaping by aid of their foot 

 (Cardium, Tellina, etc.); some are crawlers (Cyclas, Lasaea), and some 

 swimmers, by flapping the valves of their shells (Pecten, Lima). 



The Lamellibranclis are found in all parts of the world to the 

 number of more than 5000 species. They have been found to a 

 depth of 2900 fathoms. They have a wide distribution in the 

 earlier periods of the earth's history, being known since the Silurian. 

 Their fossil shells are most excellently preserved, and they are of the 

 greatest importance as characteristic fossils for the determination of 

 the age of formations. 



Order 1. PROTOBRANCHIATA. 



The gill- filaments are plate-like and not reflected, and the mantle-cavity is not 

 divided into tivo parts. 



The mantle has an hypo-branchial gland external to each gill ; the foot has a 

 ventral plantar surface, and the byssus gland is but slightly developed. The 

 nervous system has a distinct pleura] ganglion ; the otocysts are connected with 

 the surface by a tube. The sexes are separate, and the genital glands open into 

 the inner end of the kidney-tube. 



Fam. 1. Nuculidae. Palps large, with posterior appendage (Fig. 263). 

 Nucula Lam., heart dorsal to rectum; Lcda Schumacher, heart traversed by 

 rectum, mantle with two siphons; Yoldia Moll. 



Fam. 2. Solenomyidae. The palps are not bilobed ; in each gill one row 

 of filaments is directed dorsally, the other ventrally. Mantle lobes fused, 

 having a single posterior opening, and one anteriorly for the foot, tiulenomya 

 Lam. 



Order 2. FILIBRANCHIATA. 



The gill-filaments are reflected, and united by ciliary junctions. The foot 

 usually witli a ivell-developed byssus apparatus. 



Sub-order 1. ANOMIACEA. 



Asymmetrical ; posterior adductor large ; heart dorsal to rectum, and causing 

 projection into mantle-cavity ; the reflected limbs of the inner filaments are 

 fused across the middle line with their fellows; genital glands open into kidney, 

 and that of right side extends into mantle. 



Fam. 1. Anomiidae. Anomia L., byssus calcified, passing through a hole in 

 the right valve. Placuna Bruguiere. 



Sub-order 2. ARCACEA. 



Symmetrical ; mantle open ; both adductors well developed ; gills without 

 inteiiamellar junctions; renal and generative openings separate. 



Fam. 2. Arcidae. Edges of mantle with compound pallial eyes. Pectunculus 

 Lam., Limopsis Sassi ; Area L., heart dorsal to rectum (Fig. 261). 



Fam. 3. Trigoniidae. Trigonia Bruguiere. 



