48 Dr. F. Miiller on the Transformations of the Porcellanae. 



obliquely forwards and downwards ; in the common Porcellana 

 they are beset beneath with an entire series of small spines, and 

 exceed the carapace in length ; in Porcellina they attain more 

 than three times the length of the carapace. Thus, in the latter 

 species, the carapace of the newly-hatched young, with its pro- 

 cesses, is twice as long as that of the mother. 



Besides this remarkable carapace, the only structure which 

 differs remarkably from other young Crabs is that of the last 

 segment, which is dilated into a fin. It is well known that the 

 last segment of the larvae of Crabs is extended on each side into 

 a horn, often of considerable size, and that in the emargination 

 between these horns, three short plumose bristles usually stand 

 on each side. In the Porcellanae the lateral horns are replaced 

 by inconspicuous spines, and the middle part projects so far 

 between them that the whole tail acquires nearly a rhomboid 

 form. In Porcellina this is particularly elongated, more than 

 twice as long as broad. On each of the two posterior sides of 

 the rhombus there are five long plumose bristles. (An inter- 

 mediate form, but approaching most closely to the Porcellana, 

 is presented by the tail of the young Paguri.) 



In all other respects, in the structure of the eyes, antennae, 

 mouth, and feet, the young Porcellana agree entirely with the 

 young Crabs, and exhibit no greater difference from them than 

 the latter do among themselves. 



In both, the anterior antenna? (PI. I. fig. 5 a) are not jointed, 

 and have a strong nervous knot in the vicinity of their apex, 

 from which, besides a few minute bristles, two (three in Porcel- 

 lina) longer peculiar filaments issue. These are of uniform 

 thickness, or rarely a little tapering ; they terminate in a rounded 

 extremity, and are further distinguished from other bristles by 

 their very delicate outline and dull turbidity. The same fila- 

 ments, however, recur on the anterior antennae of young Bopy- 

 ridce (they are especially distinct in Entonitas cancrorum, n. sp.) 

 and Cirripedes ; in the latter they spring singly from a minute 

 basal joint close to the eye. 



The posterior antennae (fig. 5 b) in Porcellina stellicola already 

 exhibit a great resemblance to those of the mature animal (fig. 2) 

 — the same inflated basal joint with the well-known opening of 

 the still problematical sensorial organ, the same acutely trian- 

 gular second joint, from the outside and upper part of which 

 issues, in the one case, a multiarticulate flagellum, and in the 

 other a simple spine-like process. The same pieces occur in the 

 same form in the other species*. 



* In the Zoea of a small Xantho, the outer antenna; (fig. 11) attain the 

 length of the frontal horn, and the future flagellum is so small as to be 

 almost imperceptible, 



