PROTEOLKPAS BIVINCTA. 605 



cement-ducts, did not essentially differ from the imaginary 

 figure given, PL 25, fig. 6. Now, at the period of the 

 metamorphosis, let us imagine that no general covering or 

 carapace was formed, except a small portion on the ventral 

 surface, round the cemented antennae. Let us further 

 suppose this remnant to be specially developed (as in the 

 case of some cirripedes) into a short trouser-like pro- 

 longation, entering the antennae ; and subsequently, in ac- 

 cordance with the almost universal laws of growth in cirri- 

 pedes, that this portion was never moulted, but continued 

 to be added to, during growth, only at its upper end. By 

 this means we should produce every leading peculiarity 

 of the Proteolepas bivincta. As this parasite lives within 

 the sack of another cirripede, and is protected by the capi- 

 tulum of the latter, we can understand, in accordance with 

 the usual admirable economy of nature, the absence of any 

 general covering for its body. We can now, also, under- 

 stand the structure and manner of growth of the two threads 

 by which it is bound to its prey ; and the connection, at first 

 so strange and perplexing, between the old pupal antennae 

 and the second segment of the thorax. I am convinced that 

 no other explanation than that here given, will accord with 

 the relations of the several parts and organs of Proteolepas. 

 Consequently, I fully believe that we here see an articulate 

 animal in which the whole of the three anterior segments 

 of the head have been, during the act of metamorphosis, 

 absolutely aborted, with the exception of a mere rudiment 

 on the ventral surface, near the anterior end, round the old 

 antennae, and which rudiment has been specially developed 

 as a covering for the two cement-ducts. As the pupal 

 antennae are, homologically, the second pair of antennae, we 

 may further infer that this modified remnant of the cara- 

 pace, investing the two threads, belongs to the third cephalic 

 segment. 



