80 ZOOLOGY. 



blood towards the margin. TJie surface is covered with vibrillge, so tliat 

 they must be used in breathing, although the general surface may be 

 adapted to this function. 



Fain. 4. Placliobrancliidce. Founded upon the single genus Placho- 

 hranchus of Van llasselt. The body has broad lateral expansions used as 

 swimming organs, and these, and also the back, are covered with branchial 

 lamella^. The expansions can be turned upwards, and their margins made 

 to meet over the back. P. oeellatus, two inches long, was found at 

 Java. 



Order 5. Crypsibranciiia (also known by the inadmissible name of 

 Tectibranchia). The branchia? are imsymmetrical, composed of leaves 

 more or less divided, and they are covered by the upturned mantle. These 

 animals are monoicous, and are comprised in the iwo families Aplysiidcß 

 and Bullidoi. 



Aplijsia {j)l. 77,ßff. 18) was named lepiis marhm.s by the ancient authors, 

 and many fables entered into its history. The shape is not much unlike 

 that of a naked snail, but the body is larger posteriorly. There are two 

 pair of tentacles, the anterior ones being the largest, and of such a form (in 

 connexion with that of the body) as to suggest the name of sea hare for the 

 animal. The branchife are in a cavity protected by an operculum 

 containing a thin corneous body, which is a rudimentary shell, and 

 have their parietes composed of interlaced fibres; they are pierced with 

 numerous small holes through which the blood may escape into the stomach, 

 and the vascular system receive the li(piid products of digestion. 



The species of Aplysia are found upon the coasts of continents and 

 islands at no great depth, some crawling upon rocks or hiding in their 

 crevices; others hiding in the mud or sand, but with the branchial siphon 

 exposed. Their food is marine plants or animals. The shell in the ditlerent 

 species varies in texture, some approaching the solid shell of Dolabella, and 

 the change is so gradual, that Hang, the chief authority upon these genera, 

 considers the latter genus merely a section of Aplysia. 



Fa?n. 2. Bullida\ The chief genera of this family are the allied Bulla, 

 with the shell external ; and BulUea with the shell internal. In Bulla {B. 

 j}hi/ds^ j)l. 7G,ßg. 3) the tentacles are wanting, the shell is thin in texture, 

 subglobular, more or less completely enrolled, and having the aperture the 

 entire length of the shell. 



Order 6. IIypobranchia (also incorrectly named InferohrancMci). This 

 order is named from the position of the branchiaj under the margin of the 

 inantle. It contains three families, represented by the genera Phyllidia^ 

 Pleurobranchus^ and UmhreUa. 



Pam. 1. PhylUdiidfje. Di/plnjllidia {pi. 7-i-,ßfj. 19) is oblong oval, with 

 a coriaceous rough integument, forming a prominent lateral border. The 

 • branchia? surround the body, and are in the form of transverse lamella?; there 

 are four tentacles, two of whicli are superior and retractile. 



Pam. 2. Pleiirobranc/ndce. In Pleurobranchus the branchia? resemble a 

 fringe formed of laminre, on each side of a medial branch placed in a groove 

 upon the right side between the mantle and foot. The animal is shaped 

 284 



