468 THE ANIMAL KINGDOM. 



It is spirally coiled in Limacinidae, in other Thecosomata symmetrical. 

 The larval shell of Cymbulidae, however, is spiral. 



The epidermis is ciliated in Clio. There are cutaneous glands, pigment 

 cells, and calcareous concretions in the integument. Tiedemannia has a 

 rich development of chromatophores. 



The nervous system consists of the cerebral, pedal, pleural, and visceral 

 ganglia. They are concentrated round the oesophagus, especially in Theco- 

 somata. The otocysts are closely attached to the pedal ganglia. The 

 structure of the eyes on the nuchal tentacles is unknown. A right osphra- 

 dium appears to be present in Thecosomata, and in Pneumodermon among 

 Gymnosomata is innervated in the typical way. 



The oral aperture is armed with various tentacle-like processes, which 

 are inserted in Gymnosomata within its margin. Clio has three pairs, the 

 surface of which is covered with epithelial papillae, each furnished with a 

 sense-cell, and giving exit to the ducts of unicellular glands, which nearly 

 fill the central cavity of each process or ' cephaloconus.' They act as 

 organs of adhesion in securing the prey. Pneumodermon has a pair of 

 arms, which bear pedunculate muscular suckers on their oral faces. Tentacle- 

 like processes are found near the mouth in some Thecosomata, but their 

 structure does not appear to have been investigated. The oral cavity of 

 Clio and Pneumodermon contains a pair of eversible sacs armed with 

 chitinoid hooks. A pair of chitinoid jaws is generally present, in addition 

 to the radula. Salivary glands are absent or rudimentary in Thecosomata. 

 There is a long oesophagus, a stomach, and intestine. The anus is on the 

 left (?) side in Hyaleidae, Cymbtdia, and Tiedemannia, on the right in all 

 other Pteropoda. A liver is well developed in Thecosomata, but in the 

 Gymnosomata is represented only by caecal processes of the stomach walls. 

 The auricle is not sharply marked off from the blood sinuses. Its aperture 

 into the ventricle is guarded by valves. The ventricle is well-developed, 

 and gives off an aorta, which divides into an intestinal and cephalic branch. 

 Its ultimate ramifications open into a system of irregular coelomic sinuses. 

 The general surface of the body is respiratory in Gymnosomata. Pneumo- 

 dermon, however, possesses three contractile and richly ciliated processes at 

 the apex of the visceral dome, in and out of which the blood passes. A 

 semilunar series of folds within the branchial cavity on the body wall of 

 Hyalea, and a transverse shield-shaped ciliated area on the wall of the 

 mantle fold in the same Pteropod and some other Thecosomata, have had a 

 respiratory function assigned to them. The nephridium opens near the 

 anus. It is saccular, and in some, probably in all instances, opens into the 

 pericardium. Unicellular glands open into it in Clio, and in the Hyaleidae 

 its walls are said to be spongy and opaque. There is an hermaphrodite 

 gland, an hermaphrodite duct, which may be dilated near its termination, a 

 receptaculum seminis, and albuminiparous gland. The hermaphrodite 



