80 ZOOLOGY. 
blood towards the margin. The surface is covered with vibrille, so that 
they must be used in breathing, although the general surface may be 
adapted to this function. 
Fam. 4. Plachobranchide. Founded upon the single genus Placho- 
branchus of Van Hasselt. The body has broad lateral expansions used as 
swimming organs, and these, and also the back, are covered with branchial 
lamelle. The expansions can be turned upwards, and their margins made 
to meet over the back. PP. ocellatus, two inches long, was found at 
Java. 7 
Orper 5. Crypsrprancuia (also known by the inadmissible name of 
Tectibranchia). The branchiz are unsymmetrical, composed of leaves 
more or less divided, and they are covered by the upturned mantle. These 
animals are monoicous, and are comprised in the two families Aplysiide 
and Bullide. | 
Aplysia (pl. T7, fig. 18) was named lepus marinus 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 branchiz 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 liquid 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 different 
species varies in texture, some approaching the solid shell of Dolabella, and 
the change is so gradual, that Rang, the chief authority upon these genera, 
considers the latter genus merely a section of Aplysia. 
Fam. 2. Bullide. The chief genera of this family are the allied Bulla, 
with the shell external; and Bulla with the shell internal. In Bulla (B. 
physis, pl. 76, fig. 8) 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. 
Orver 6. Ifyposrancuta (also incorrectly named L/nferobrancia). This 
order is named from the position of the branchiee under the margin of the 
mantle. It contains three families, represented by the genera Phyllidia, 
Pleurobranchus, and Umbrella. 
Fam.1. Phyllidiude. Diphyllidia (pl. T4, fig. 19) is oblong oval, with 
a coriaceous rough integument, forming a prominent lateral border. The 
branchiee surround the body, and are in the form of transverse lamelle; there 
are four tentacles, two of which are superior and retractile. 
Fam. 2. Pleurobranchide. In Pleurobranchus the branchiz resemble a 
fringe formed of laminee, on each side of a medial branch placed in a groove 
upon the right side between the mantle and foot. The animal is shaped 
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