THE HETEROrOD MOLLUSKS. 251 



the tongue or radula has highly characteristic teeth, which 

 serve these rapacious animals to seize their prey. The in- 

 testine runs straight back from the month, and after mak- 

 ing one or two coils ends in the vent. The excretory organs 

 open near the aims ; the contractile tube opens internally 

 into the pericardial cavity, and resembles in form and posi- 

 tion the excretory organ of the Pteropoda. The circula- 

 tion is imperfect, the blood passing from the wide sinuses 

 of the body to the ventricle of the heart. From the auricle 

 .springs the aorta, which subdivides into several branches 

 that open freely into the body-cavity. The circulation can be 

 easily watched, owing to the transparency of the body. The 

 -aeration of the blood is effected partly through the skin, 

 partly through gills, except in a few species. The branchiae 

 are cither thread- or leaf-like ciliated appendages, which 

 may either be free or enclosed in the mantle-cavity. The 

 .sexes are distinct. The males can be readily recognized by 

 the large copulatory organ, which hangs free on the right 

 .side of the body. The sexual glands fill the posterior por- 

 tion of the visceral cavity, and are partly imbedded in the 

 liver. The oviduct is complicated by the presence of an 

 albumen gland and a receptaculum fteminis. It opens on 

 the right side of the body. 



o / 



The Heteropods are exclusively marine, but are found in 

 all quarters of the world. The number of species is small, 

 and there are two orders only the Pterotraclieidce Avith a 

 small or no shell and free gills, and the Atlantidcs with a 

 large coiled shell and gills placed in the mantle. Ptero- 

 iracliea (Firola] coronata Forsk. is found in the Mediter- 

 ranean, and on account of its transparency has often been 

 investigated. The Heteropoda live together in large num- 

 bers, and feed on small animals. 



The eggs are laid in cylindrical strings, which soon break 

 up into numerous pieces. The segmentation of the yolk is 

 complete but irregular. The embryo rotates within the egg 

 during the veliyer stage, when it lias two distinct sails, or 

 lobes of the velum, and a ciliated foot with an operculum. 

 In this form it leaves the egg. The velum enlarges and 

 lorms several divisions. The otocysts, eyes, and tentacles are 



