Crangon and Galathea. 49 



10th to 14th somite — Ambulatory organs, or chelipeds (the pereiopeda of 

 C. Spence Bate).* 



15th to 20th somites — Natatory organs, frequently also respiratory. 



2 1st somite, or telson, generally unappendiculate, sometimes bearing nata- 

 tory plates; and occasionally, as in adult states of some Isopoda (ex. gr. Ligia), 

 and in immature stages of some decapoda (ex. gr. zoe of Pirimela denticulata), 

 respiratory. All these appendages, except, possibly, the first to fourth pairs 

 inclusive, are arranged on one general type, being septemarticulate, one or more 

 of the articulations being furnished with an appendage, or appendages, which 

 are often multiarticulate. 



We further find that in Decapoda, Isopoda, Amphipoda, and probably also 

 in the other groups (my own researches in these latter have not been sufficient 

 to enable me positively to assert it), that the fourth, or mandibular somite, 

 either with or without portions of the ocular and auteunal somites, constitutes 

 a more or less extensive shield, beneath which the gastric and oral appendages 

 are placed ; the " carapace" of Decapoda, as the author believes, homologizing 

 with the " head," or cephalon, in Isopoda and Amphipoda. 



Here it should be noted that some of our writers in Carcinology adopt a 

 different view of the primary divisions of the somites : they regard the anterior 

 seven somites as forming a distinct division, equal in value to the succeeding 

 seven, and to the posterior seven. Thus they divide the cephalo-thorax into a 

 head, or cephalon, and a thorax, or pereion. Spence Bate, by whom the latter 

 terms are used, has ably advocated this view ; but I cannot subscribe to it. 

 Dana, in his treatise on Crustacea, has given some strorg arguments in favour 

 of the opinion here adopted, to which may be added, that while the appendages 

 of the first four somites in all the Crustacea at present known are constant in 

 their oflices (for I must dissent from Professor Huxley's explanation of the 

 cheliform arms of the extinct Pterygotid^, the evidence as yet adduced not 

 being sufficient to prove that they were not maxilliE, or maxillipeds, rather than 

 antennae) ; the moment we pass the boundary of the 4th somite, the appen- 



* Eeport on British Edriophthalma, Brit. Ass. Rep., 1855, pp. 35, &c. Of the terms there 

 proposed several are adopted in this Paper. 



