106 TECTIBRANCHES. HETEROPODS. 



the branchiae are found partially concealed under the margin 

 of their cloak, where they form a series of filaments nearly 

 encircling the body. The order contains two genera only 

 the Phyllidia and Diphyllides which are found in the 

 Indian Sea and in the Mediterranean. 



4. TECTIBRANCHES. The branchiae, in the shape of leaves 

 more or less divided and non-symmetrical, are placed on the 

 back or side, and are covered with a fold of the cloak, which 

 almost always contains a shell more or less developed. In 

 the character of the branchiae they approach the Pectini- 

 branches ; but the Tectibranches, like the three first orders, 

 are bisexous, although, in order to propagate the species, 

 the animals require to act as if the sexes were distinct. 

 The Aplysia, in which the branchiae are dorsal and covered 

 with a horny plate, may be considered as the typical genus 

 of the order. In Pleurobranchus the branchiae are unilateral, 

 simpler, and more fully exposed. The Aceres have many 

 characters which closely ally them to the Aplysiae, but the 

 shell is calcareous, frequently external, and always convolute ; 

 so that the pure Conchologist willingly allows them a promi- 

 nent place in his cabinet under the name of Bullae, while he 

 disdains to notice their shelless confederates. 



5. HETEROPODS. These carry the branchiae upon the 

 posterior part of the back, where they form a row across it 

 of little plumose tufts, and are, in some of them, protected, 

 as well as a portion of the viscera, by a symmetrical shell. 

 What best distinguishes the order is the compressed foot of 

 its members fashioned into the form of a thin vertical fin, on 

 the margin of which, under the head, we often perceive a 

 small sucker the sole vestige of the horizontal foot charac- 

 teristic of the class. The Heteropods are eccentric in their 

 appearance, natives of the Mediterranean and Indian Ocean, 

 and frequently found floating with the Gulf-weed, on which 

 they attach themselves by their sucker. The principal 

 genera are the Carinaria, remarkable for its beautiful glossy 

 navicular shell ; the Atlanta, with a shell rolled up spirally, 

 and in one of which Lamanon believed he had found the 

 original of the Ammonites ; and the Firola, which has no 

 shell, but in whose singular shape fancy may trace a like- 

 ness to some ancient ship or New Holland war-canoe, the 

 proboscis being raised and bent like the prow, while a thread- 

 like filament, prolonged from the tail, simulates the oar that 

 serves for its guidance. 



6. PECTINIBRANCHES. As the name implies, the branchiae 

 are pectinate, viz., composed of fringes arranged in parallel- 

 ism like the teeth of a comb, and forming a leaf concealed 



