614 CLASS °X: 
mouth is more forward, there are two simple eyes, and the smaller 
pair of antenne has disappeared. In the third stage, the larva is 
much compressed, nearly of the shape of a Cypris, and the thorax. 
and limbs hidden and enclosed by the carapace elongated back- 
wards. The part of the head bearing the antenne is longer and 
larger than the rest of the body. The antennz are large and con- 
spicuous, consisting of three segments, of which the second (a 
sucking disc) is much the largest, the third very small. The 
antenne serve for walking, but their principal use is to attach the 
larva, the attachment being at first voluntary, but soon becoming 
permanent. There are now two large compound eyes close behind 
the base of the antenne. The mouth, as in mature cirripeds, is 
situated on a slight prominence in front of the thoracic limbs. It is 
within the carapace, and still rudimentary. The thorax consists of 
six seements with six pairs of feet, each with a pedicle bearing two 
arms of two joints. The abdomen is small, but with three seg- 
ments, of which the second is the longest, and bears two small 
appendages between which the anus is situated. The bivalve shell | 
and compound eyes of the larva are first moulted: the antenne not 
at all. The young cirriped is closely packed within the larva, and 
there are two rudimentary eyes posterior to the cast-off eyes of the 
larva. They are situated beneath the integument on the upper 
part of the stomach!.] Scarcely less strange are the changes which 
most of the decapod crustaceans undergo. The early states of short- 
tailed crustaceans have been frequently recorded in systematic 
works as distinct genera. Thus the genus Zoe Bosc, with large 
eyes and a long bent beak and a recurved hook on the back, is 
founded merely on individuals in the first period of life of Cancer 
and Hyas*. These animals have then a long tail, which only at 
a later period is bent under towards the breast. But many long- 
tailed crustaceans also undergo changes of form, ex. gr. Pagurus. 
In Astacus marinus the feet are at first provided with a jointed fila- 
ment, which corresponds to the flagrum of the auxiliary jaws. Of 
all the decapod crustaceans hitherto investigated Astacus fluviatilis 
1 Darwin, Lepadide, pp. 8—25. 
2 See RatHKE Reisebemerkungen aus Scandinavien, Neueste Schriften der natur- 
forschenden Gesellsch. zu Danzig, 111. 4tes Heft, 1842, Tab. tv. The discovery of the 
singular change of forms in Decapods was first made by JoHN THompson. RATHKE, 
relying on his own investigations in Astacus fluviatilis, at first doubted, nay, even 
