PTEROPODA AND GASTROPODA. 



537 



Hyalsa. 



system in this blind and non-tentaculate pteropod partakes of 

 the simplicity of that in the bivalves ; inasmuch as there is no 

 ganglionic enlargement of the upper part of the oesophageal loop. 

 The chief centre (^fig. 197, r) consists of a large flat subquadrate 



suboesophageal mass, from the an- 

 197 terior angles of which proceed 



the chords encircling the gullet, 

 and some nerves to the fins : the 

 other nerves to the fins and those 

 to the rest of the body are given 

 off from the posterior angles of 

 the great ganglion.* The acous- 

 tic vesicles are in contact with the 

 fore part of this ganglion ; they 

 are large, and have otolites, kept 

 in motion by the vibratile cilia 

 of the inner surface of the vesicle. 

 The lingual teeth are arranged 

 in transverse rows of three in 

 each ; they are long and recurved. 

 The oesophagus (y,Jigs. 196, 197) 

 overlies the penis ; it dilates into 

 a kind of crop {fig. 197, tv), which conducts to a cylindrical gizzard 

 (ar) : both these have their inner surface produced into longitudinal 

 folds (9), those of the crop {w) being more numerous than 

 those of the gizzard (x). The intestine {fig. 197, y) is a narrow 

 tube of uniform diameter, which describes two convolutions, bending 

 towards the ventral surface, in the substance of the liver {fig. 196, 

 6, X, and 197, z, z). The anus is on the right side of the neck, 

 beneath the right fin. In Sagitta the hepatic substance appears to 

 be blended with the intestinal walls. In Pneumodermon the stomach 

 is lined by a layer of small hepatic follicles. If the dissection of 

 Hyalcea be made from the dorsal aspect, as in fig. 196, 6, the heart, 

 which consists of an auricle and a ventricle, is seen on the left side 

 upon the inner border of that end of the gill, s. This branchial 

 organ is a long narrow body subdivided into small transverse 

 ciliated lamellae, and describing a curve which runs almost parallel 

 with the border of the shell-cavity, p, q,s,fig. 196, 6, and^^r. 195,4. 

 A contractile renal organ opens into the branchial cavity. 



The male and female organs are combined in the same individual 

 in all Pteropods. The ovario-testis {fig. 196, 6, to), presenting a 



* Cuvier, owing to his opinion as to the dorsal and ventral aspects of the 

 Hyalsea, describes this ganglion as the brain, CCCXXI. 



