ORDER TECTIBRANCHIATA. 43 
two tubular and cleft tentacula ; the genital orifices are before, 
and the anus behind the gills. There are four stomachs, the 
second of which is fleshy, and sometimes armed with bony 
appendages, and the third furnished internally with salient 
longitudinal lamine ; the intestine is short. 
Various species inhabit both the Mediterranean and the 
Atlantic, some of which are large, and marked with the most 
beautiful colours. (Pleuwrobranchus Peronti, Cuv., &c.) 
PLEUROBRANCH&A, Meckel. PLEUROBRANCHIDIUM, 
Blaine. 
The gills and genital orifices situated as in pleurobran- 
chus, but the anus is above the gills; the margin of the 
mantle and foot project but little, and on the fore-part of the 
former are four short distant tentacula, forming a square, that 
reminds the observer of the anterior disk of the akere. I can 
find but one stomach, which is merely a dilatation of the canal, 
with thin parietes. A multifid glandular organ opens behind 
the genital orifices. ‘There is no vestige of a shell. 
Pleurob. Meckelii (Leve, diss. de Pleur. 1813), is the only 
species known in the Mediterranean. 
APLYSIA, Linn. 
The margin of the foot turned up into flexible crests, sur- 
rounding the back, and even susceptible of being reflected over 
it; the head supported by a neck more or less long; two 
superior tentacula, excavated like the ears of a quadruped, 
with two flattened ones on the edge of the lower lip; the eyes 
beneath the former; the gills are on the back, and consist of 
highly complicated lamellz, attached to a broad membranous 
pedicle, and covered by a small membranous mantle, in the 
‘thickness of which is a flat and horny shell; the anus opens 
behind the gills, and is frequently concealed under the lateral 
crests; the vulva is before, on the right, and the penis pro- 
