54 ON THE CARAPAX AND STERNUM 



with the carapax it follows that the carapax in the young 

 Squilla pertains to the antennary and mandibulary somite 

 to these and these only. The same is true of the zoea 

 of Porcellana. The relations of the carapax in the young 

 stages of Cancer and Carcinus could not be made out ac- 

 curately, owing to the poor state of preservation of the 

 specimens at my disposal. Among the Brachyurathe ter- 

 gum of the ophthalmic somite is present as a distinct plate 

 beneath the carapax and may be exposed by cutting away the 

 rostral region of the carapax, or it may sometimes be seen 

 from behind (e.g.,Platyonychus, Actceodes, Scytta). The 

 antennulary tergum, on the other hand, seems to have dis- 

 appeared entirely. 



The sternum of the ophthalmic somite, considered by 

 Dana to be wanting among the Brachyura, is present, as 

 it appears to me, in what has hitherto been considered as 

 a portion of the antennary somite and designated the an- 

 tennary septum (compare Huxley, loc. cit., p. 296, fig. 

 76, c.). In Actceodes, figs. 4, 6 and 7, the sternum of 

 this somite is a distinct cuneiform body, wedged in be- 

 tween the rostrum and the antennary sternum, but sep- 

 arated from both by sutures. 



Its connection is more intimate with the antennary ster- 

 num than with the rostrum. The basal joints of the 

 antennae lie in contact with it, since it helps to form the 

 inner angle of both antennary orbits. Ihis wedge- 

 shaped body extends backward into the facial region and 

 furnishes the calcareous sockets for the bases of the eye- 

 stalks ; but has nothing to do with the orbital region. 

 This latter has arisen by the overgrowth of the rostral 

 region (*. e., forwards) which at the same time has been 

 forced downward into the facial area. This growth is 

 well illustrated in the series from Homarus, through 

 Lithodes, Platyonychus, Scylla and Cancer, to Actceodes. 



