292 LECTURE XXII. 



cephalic expansions may be regarded as analogous to the large arms 

 of those Acalephae : but the Pteropods manifest a much higher grade 

 of development in having a distinct head, with tentacula and jaws 

 anterior to the attachment of the wing-like expansions. In the 

 Hyalaea, the head and fins form together a large division of the body, 

 which has been compared to a cephalo-thorax ; the part containing 

 the viscera and which is lodged in the shell forming the abdomen. 



In this species the upper or dorsal valve is nearly flat, and is pro- 

 longed anteriorly beyond the ventral plate, which is convex. Thin 

 lobes of the mantle correspond with the divisions of the shell, but are 

 united together at the sides ; from which long membranous processes 

 are continued in some species. The principal part of the muscular 

 system arises by a narrow tendon from the posterior point of the 

 shell, passes forward through the abdomen between the ovary and 

 branchia, expanding as it approaches the head, where the fibres de- 

 cussate, and diverge in the substance of the fins : other strata of 

 muscular fibres decussate the preceding obliquely. 



The mouth is a small longitudinal fissure, at the apex of two 

 diverging labial eminences : it contains a lingual prominence, covered 

 by a thin uncinated horny plate. In the Clio the tongue is more 

 prominent, and is beset with transverse rows of recurved hooks ; and 

 the mouth is provided with two small lateral jaws, supporting a row 

 of unequal horny teeth. * Salivary glands coexist with the maxillary 

 organs in the Clio; they are wanting in the Hyalcea. In both 

 Pteropods the stomach is large and globular, the intestine disposed in 

 three or four coils, and terminating at the right side, and towards the 

 anterior part of the mantle. The liver is voluminous, in part conceal- 

 ing the stomach, but transmitting the bile by a single duct to the be- 

 ginning of the intestine. In the Pneumodermon the biliary secretion 

 is said to be poured into the stomach by many pores ; as, in the ace- 

 phalous bivalves. 



The cephalic expansions of the Clio are muscular, like those of the 

 Hyalcea, subserving locomotion, not respiration, as Cuvier believed. 

 The true branchial vascular network is developed in both Pteropods 

 upon the inner surface of the mantle. A heart, consisting of an au- 

 ricle and ventricle, is lodged in a pericardium, and situated on the 

 left side in front of the branchia, from which it receives the blood by 

 a large vein, and propels it by two aortas to the locomotive append- 

 ages and other parts of the head, and to the abdomen and its viscera. 

 The Hyalaea and Cleodora have no cephalic tentacles. The Cymbu- 

 lia has two small tentacula with an ocellus at the base of each. 



* Eschricht, Anat. Unters. uber die Clione borealis. 4to. 1838. 



