132 BALANIDiE. 



which bears the second pair of antennse, or of the fourth, 

 i.e. the mandibular segment; but from the distribution of the 

 nerves, he now argues that it must mainly belong to the 

 third segment. In Cirripedes, the course of the nerves leads 

 to the same conclusion ; for the whole shell, sack, and pe- 

 duncle are supplied with nerves proceeding from the com- 

 pounded ganglion, which belongs to the second and third 

 cephalic segments.* 



The whole of the head in front of the mouth, together 

 with the carapace, is, as we know, formed of three seg- 

 ments ; and each of these segments, homologically, ought 

 to consist of eight elements ; I recall to mind these facts, 

 inasmuch as the transverse separation between the peduncle 

 and capitulum in the Lepadidse, and between the basis, the 

 shell, and the opercular valves in the Balanidae, might be 

 thought to be connected with the separation of the three 

 caphalic segments. So again, as in the Balanidse the shell 

 normally consists of eight compartments, these might be 

 thought to be related to the eight elements of one or other of 

 the three segments. But I see no reason for admitting this 

 view ; and in the case of the carina and rostrum, two of 

 the most persistent and important of the compartments, 

 they exactly cover the sutures which ought to separate the 

 two tergal and two sternal elements of the segment. The 

 valves, moreover, often form many more whorls than three, 

 or the number of the true cephalic segments in front of the 

 mouth ; and in each whorl the valves tend to stand in tile- 

 like or alternate order, with respect to those in the whorls 

 both above and below, which would not be the case, if they 

 were the eight elements of the segments. 



For the true homologies of the sclerodermic plates, or 

 shelly valves, with which the external covering of Cirripedes 

 is so generally strengthened, we must, I believe, look to 

 the carapace of the Podophthalmia. In these latter, we 



* This conclusion is supported by the structure of Protcolepas : in this 

 Cirripede there is not a vestige of a carapace, and as the whole of the animal in 

 front of the mouth is almost utterly aborted, being reduced to a mere covering 

 to the two cement-ducts, and as, on the other hand, the mouth with the man- 

 dibles, though peculiarly modified, is not at all aborted, there is a strong pro- 

 bability, that the abortion of the carapace is connected with the aborted state 

 of the three anterior cephalic segments ; and that the carapace in its origin is 

 not any way related to the fourth or mandibular segment. 



