572 LECTURE xxn. 



can be clearly seen. Next, the auricle and ventricle are formed by a 

 constriction of the pulsating tube ; afterwards the pericardium and 

 two vessels are visible. The globules in the fluid sent into the body 

 by the caudal vesicle go towards the heart. The embryo slug has 

 now an extensile proboscis, which is formed by the lobes below the 

 tentacles, and which afterwards disappears. A peculiar glandular 

 body attached to the parietes of the embryo-sac is compared to the 

 transitory kidney, or Wolffian body in the vertebrate embryo. At a 

 short distance from the anus there projects a caecum, which after- 

 wards becomes the urinary sac. The nervous system is at first a 

 tubular ring containing fluid ; it then swells out into the super- and 

 sub-oesophageal masses, and gets neurine in the interior of the sheath. 

 The acoustic vesicles appear above the subcesophageal mass. 



The ocular tentacles appear for a long' time as rounded and later- 

 ally compressed fins, by the side of the vitelline sac ; they may be 

 the homologues of the vela of the pectinibranchiate embryos. 



The caudal vesicle disappears before the young is excluded from 

 the egg ; its contractions go on after those of the heart have begun ; 

 and thus there are two currents, one of blood in vessels through the 

 viscera, and the other of fluid outside of and bathing the viscera. 



The pulmonary sac is formed by a simple depression at first, which 

 deepens as the heart and vessels become developed, and the vascular 

 network spreads over its inner surface. The generative organs are 

 formed after exclusion. 



Class PTEROPODA. 



Encephalous moUusks, with wing-like fins from the sides of the 

 head or neck. 



Order Thecosomata. 

 With an external shell : head indistinct. 

 Family Hyaleidce. Shell symmetrical. 



Genera Hyalea, Cleodora, Cuvieria, Eurybia, Cymbulia, Tiede- 

 mannia. 



Family Limacinidce. Shell spiral. 



Genera Limacina, Spirialis, Cheletropis. 



Order Gymnosomata. 



Without a shell : head distinct : fins attached to the neck. 

 Family Cliidce. 

 Genera Clio, Pneumodermon, Pelagia, Cymodocea. 



