Plate 84. 
MOLLUSCA. OPISTHOBRANCHIA. PTEROPODA. ACEPHALA. 
The Opisthobranchia are a species of marine slug of an elon gated form and furnished 
with a variety of appendages, which serve as gills They are found among the seaweed 
under water in the vicinity of the coast, and are mostly very gorgeously colored. The 
animals are all hermaphrodites, and their numerous eggs are clustered together in a gela- 
tinous mass, the young emerging from the same in a larva form, which is distinguished 
by the presence of a spiral shell. After a time the larva casts aside its shell and begins 
to use its foot, which becomes broader by degrees and thus imparts to the animal its per- 
fected form. ; : : f 
1. The Aplysia Depilans is a marine slug with a head bearing four feelers, the pos- 
terior pair of which resemble the ears of a hare. The creature cannot swim, but is only 
able to creep about, and if touched it emits a fluid of a dark violet tint which, owing to 
its poisonous properties, serves as a means of defence. The Aplysia is very plentifully 
found on the coasts of the South of Europe, and attains a length of six inches. 
2. The Pleurobranchus Peronii is indigenous to the south sea. Its egg-shaped body, 
viewed from above, resembles a flattened disc with an arched back, and it is so soft that 
its form is easily changed, which is very often the case. The animal is found under the rocks 
at small depths, and on being touched it rolls itself together and lets itself sink. 
8. The Doris Pilosa bears two conical warts or excrescences on its back, of varying 
size, and two large feelers, the surface of which is disposed in diagonal folds. It varies in 
color from brown to yellow, and is found among the wrack and seaweed, more especially 
in the Bay of Kiel in Holstein. 
4. The Dendronotus Arborescens is one of the most beautiful of marine animals. Its 
delicate flesh-colored body is set all over with warty excrescences, resembling the in- 
equalities of the bark of a tree, which contain the organs of respiration. This genus is 
very plentifully found in the Northern seas, especially in the Bay of Kiel, where it creeps 
about among the seaweed. 
5. The olis Papillosa is distinguished by the numerous nippleshaped excrescences 
with which its body is fringed on either side, and which contain a sting, serving as a 
weapon of defence. The animal attains the length of two inches and creeps along very 
slowly. It is able to roll itself together like a hedge-hog. — 
The Pteropoda have nothing in common with the Opisthobranchia. In shape they differ 
very much, according to the varying form of the wing-like appendages, which they are able 
to move to and fro, like the wings of the butterfly. The Pteropoda are found in the open sea. 
6. The Hyalea Marginata is found in the Mediterranean. Its body is enclosed in 
a thin horn-like shell, into which the wingshaped fins can be withdrawn. {ts lays its eggs 
enclosed in transparent shells, which float about in large clusters. 
7. The Clio Boreahs has a body somewhat resembling a carrot in shape, with a 
plainly defined head and a pair of wings attached to the neck. It is very commonly found 
in the arctic seas. — 
8. The Carinaria Cymbium belongs to the family of the Heteropoda, and has an elongated, 
semi-transparent body, the back of which bears a small, thin shell, resembling glass in ap- 
pearance, while its under side is furnished with a fin or keel, which serves as an organ of 
propulsion. It is a very helpless creature and is found in the Mediterranean, where it is 
exposed to the attacks of a multitude of enemies among the fishes and crustacea, — 
The Mussels are soft, headless creatures, whose body is enclosed between two cal- 
careous shells, which when closed fit tightly together. The body consists of a mantle, 
i. e. the portion lying next to the shells and enclosing the organs, two pairs of branchie, 
-and the wedge-shaped foot, which the animal can protrude from the shell and use as an 
organ of locomotion. 
9. The fresh-water Mussel, Anadonta Cygnea, is found in stagnant, muddy water, and 
is distributed all over Europe, its size varying very much, being sometimes as much as 
7 inches in length. 
10. The Unio Pictorum is a very common fresh-water species, its form varying accord- 
ing to the depth of the water and the nature of the bottom. A kindred species, the Unio 
Margaritifer, which is found in clear, smoothly flowing brooks, yields pearls, and is on 
this account cultivated and preserved in Bavaria, Saxony and some other parts of Ger- 
many, although the number and quality of the pearls thus obtained are but trifling. 
11. and 12. The Edible Mussel, Mytilus Edulis, fig. 11, showing the animal in its shell, 
fig. 12, the empty shells, is a wellknown shell-fish, found in large quantities in the Baltic 
and North Sea, and constitutes an important article of food among the lower classes. 
With the byssus, or beard, the animal is able to attach itself so firmly to the rocks or 
other fixed objects as to defy the force of the strongest corrent to dislodge it. 
ea ERAT Sem tate: dia 
