Survey of the Animal Kingdom 311 



Class 3 — Pelecypoda (pel e -sip' o da) (Gr. pelekos, hatchet; pons, foot) or 

 class Lamellibranchiata (L. lamina, thin sheet; bronchiatus, having gills). — The 

 mantle cavity has gills which are usually lamellate (sheetlike). The muscular 

 foot, which is used for locomotion, somewhat resembles a hatchet. The calcareous 

 shell consists of paired valves, which are secreted by the bilobed mantle. The 

 pelecypods are bilaterally symmetrical. They are all aquatic and most of them 

 are marine. None possess head, tentacles, or eyes. 



Examples: Mussels, clams (Figs. 118, 120, 121), oysters, scallops, and ship- 

 worms (Fig. 122). 



Olfactory tentacle 



Pharynx. 



PeniV . 



Salivary duct 



stomach.. ^ _ai 



Salivary qland - 

 TlaqeUum . | 



Jeminol 



receptacle 



Vas deferens 



Oviduct 



Albumen 



Cjland 



Ovotest\s_ t 



Liver 1 



Eye tentacle 



Brain 



^ 9^n{fcal pore 



Finger q/ond 



p Dart sac 



Maqina 



Anal opening 



Ureter opening 



^Pulmonary 

 ^ Veins 



Lang 



.:^^i^U^ -Auricle and 

 z^^j^lf- -Ventricle of 

 r^±-W heart 



'^ kidney 



\ Ureter 



Aorta 

 Intestine 

 Foot 



Fig. 117. — Anatomy of the snail [Helix pomatia) , the roof of the pulmonary 

 sac cut at the left and turned to the right; the pericardium and visceral sac 

 opened and the viscera or internal organs somewhat separated. The finger gland 

 is also called the accessory or mucous gland. 



Class 4 — Cephalopoda (sef al -op' o da) (Gr. kephale, head; pons, foot). — 

 The head and foot are fused to form a tentacled secondary head. Suckers are 

 present on the tentacles. They possess external bilateral symmetry. The nervous 

 system is well developed in the head. 



Examples: Squid (Fig. 123), octopus (Fig. 124), cuttlefish, and nautilus. 

 Class 5 — Scaphopoda (ska -fop' o da) (Gr. skapho, a boot; pous, foot). — 

 These elongated marine types possess a trilobed foot for boring in the sand. The 



