. 

 614 CLASS X. 



mouth is more forward, there are two simple eyes, and the smaller 

 pair of antennae 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 antennas is longer and 

 larger than the rest of the body. The antennas 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 

 antennas 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 antennas. 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 segments 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 antennae 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 1 .] 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 z . 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, Lepadidce, pp. 825. 



2 See KATHKE Reisebemerkungen aus Scandinavien, Neueste Schriften der natur- 

 forschenden Gesellsch. zu Danzig, HI. 4tes Heft, 1842, Tab. iv. The discovery of the 

 singular change of forms in Decapods was first made by JOHN THOMPSON. RATHKE, 

 relying on his own investigations in Astacus fluviatttis, at first doubted, nay, even 



