128 INVERTEBRATE ZOOLOGY. 



keeping close to the under surface of the shell, cut the two adductors, one 

 near the anterior, and one near the posterior end, lying towards the dorsal 

 side. Then remove the right shell entire, cutting through the hinge ligament. 

 Clean and dry the shell removed, and when dry notice :— The scars for the 

 anterior and posterior adductor muscles ; near these, smaller scars for foot- 

 muscles, protractors and retractors ; a line parallel to the edge of the shell 

 where the mantle was attached, the paZZmHrne, which curves in posteriorly 

 to form tlxe pallial sinus for the reception of the siphon : dorsally, the hinge, 

 where the two shells are articulated. Lift up the exposed mantle and notice 

 the relations of siphons, adductors, and foot. Find the opening for the foot. 

 The large ventral chamber now exposed is the infra-bvanchial chamber com- 

 municating with the exterior by the brachial siphon. Prove this. A large 

 rounded portion of the body with the foot attached, hangs free in this cavity, 

 upon each side of which are two delicate gill plates. In front of these are 

 four flat oral tentacles surrounding the mouth. Remove the remains of the 

 mantle and other overlying parts from just beneath the hinge and find the 

 heart, perforated by the intestine. Notice the pericardial chaynher in which 

 it lies. Trace out the intestine posteriorly to the anal opening around the 

 posterior adductor muscle. With which siphonal oj^ening does it communi- 

 cate ? Cut through the membrane at the base of the gills, and thus exjiose 

 the sirpra-hrauchial or cloacal chamber. Notice the perforations in its floor 

 for the water which comes from the gills. 



I O I ■ Dissection of special systems. The above section serves mainly to 

 give the general topography of the parts, and the location of the cavities. 

 As both shell and mantle are formed from an integumental fold, the infra- 

 branchial chamber is outside the true body and hence not comparable to a 

 coelom. The same may be said of the supra-branchial chamber. The pendu- 

 lous body mass is solid and consists of digestive tract, hepatic, and reproduc- 

 tive glands, packed firmly together and enclosed in a layer of muscle fibres. 

 The true coelom is thus limited to the pericardial chamber, which contains the 

 inner openings of the nephridia. After this general view, the anatomy of 

 each system shoukl be studied, making use of different aspects and exjjosures, 

 using fresh specimens when necessary. For some systems, a complete ven- 

 tral or dorsal view is useful, obtained by removing both shells and pinning 

 the animal out as desired. 



1. Respiratory system. Each gill-plate is composed of two layers, con- 

 nected by frequent commissures, bearing between them water-pores. The 

 blood circiilates through blood spaces in the commissures and the lamellae 



