578 BIOLOGY. 



for the first time distinct and individualized, and are also 

 a characteristic organ of the class ; so also do they serve 

 as bases of division, and the Snails may be divided into 

 those which are Androgynous or bisexual, and those 

 with separate sexes, i. e. Dioecious. 



Order 1 . Protozooid Snails Androgyni. 



3496. The male and female sexual parts united toge- 

 ther in a single individual, branchiae ramuliform, occurring 

 either as filaments, leaflets or ramules, freely situate upon 

 the naked body, or as a network lodged within the pallial 

 cavity and surrounded by a shell. 



The Nudibranchiate species live in the sea, those with 

 hollow branchiae or the Pulmonea in the air ; the former 

 feeding mostly upon animals, the latter upon plants. 

 Fam. 1. Infusorial Snails Notobranchiata, or Tritonia. 



Body gelatinous and membranous, cylindrical and 

 naked, devoid of shell, with branchial filaments or ra- 

 mules disposed in two rows upon the back. 



Here belong the Tritoniae and Dorides. Their body 

 is muscular ; the tentacula not retractile ; the male 

 sexual parts open in company with the female upon 

 the right side of the neck, as in the higher organized 

 Snails. All inhabit the sea. They prefigure or typify 

 the Salpae. 

 Fam. 2. Polypary Snails Pleurobranchiata, or Patella. 



Body and sexual parts as in the preceding family, 

 the branchiae, however, occurring as ramules or leaflets 

 upon the sides of the body, are more or less covered. 



Here belong the Phyllidiae, Schussel- and Schild- 

 schnecken. They are the antetypes of the Ascidiae. 

 Fam. 3. Acalephan Snails Dictyobranchiata orLimacidte. 



The branchiae form a rete or network within the 

 pallial cavity, and respire the moisture of air ; mantle 

 and viscera are mostly surrounded by a shell ; the body 

 is therefore bipartite, being separated into a splanchnic 

 or visceral body and a foot with head. 



Here belong the Air-breathing Snails, both land as 

 well as fresh-water species. They typify the Cirripedia. 





