1866.] WOODWARB STEUCTURE OF XIPHOSFRA. 33 



difference of opinion, then, is as to the anterior 14 segments — how 

 many are cephalic and how many thoracic ? 



Dr. Dana* considers the head and thorax in the Crustacea to be 

 always blended in a cephalothorax ; but although this view is correct 

 as regards the Decapoda, its apphcability does not hold good in all 

 the other orders of the class. 



Professor Huxley f considers that the division in the Podophthalmia 

 is marked by the cervical fold and by the sudden change in the cha- 

 racter of the appendages of the sixth and seventh somites, and an 

 equally marked similarity between the latter and those of the eighth 

 and ninth somites. 



According to this view, then, we shall have six cephalic, eight tho- 

 racic, and six abdominal somites; for Professor Huxley views the telson 

 as a median appendage, and not as a true segment. I do not find, how- 

 ever, that this view is generally adopted by other carcinologists. 



Assuming the old view of the Crustacean type to be correct, i.e. 

 seven somites for the head, seven for the thorax, and seven for the 

 abdomen (counting the telson as a terminal segment, on which point 

 there is no doubt some uncertainty, seeing that in lAmulus it is deve- 

 loped subsequently, though this is not the case in the Decapoda), we 

 shall find that the first thoracic somite is united to the cephalon in 

 Limulus (bearing the operculum), that there are only five other ap- 

 pendages to the thorax, making six with the operculum, and that 

 there are no appendages to the abdomen. 



Of the appendages belonging to Belinurus, Prestwichia, Hemi- 

 aspis, Pseudoniscus, Exapi7iurus, and Bunodes, I cannot speak with 

 certainty, but can only infer, from the general similarity in their 

 bodies of the first two genera to Limulus, and of the last four to 

 Euryjpterus, that they were constructed upon those types. 



Turning from Limulus to Pterygotus, we find that Professor 

 Agassiz regards the antennary system as altogether wanting (see 

 ante, p. 29) ; but in this we cannot but think he is mistaken. Profes- 

 sor Huxley regards the chelate organs on the head of Pterygotus as the 

 antennse ; and this opinion agrees well with subsequent discoveries 

 in the fossil forms on the one hand, and with the nature of these 

 organs in the recent Limulus. The antennules undergo no sexual 

 variation in the Hving King-crabs, it is invariably the antennaG that 

 are modified. (See PL II. fig. 5, antenna of male King-crab ; fig. 6, 

 antenna of female of same.) 



Assuming the antennules to be absent, and the first pair of organs 

 after the eyes to be the antennae, we shall find the other organs to 

 correspond most completely with Limulus, and that one thoracic 

 segment is united to the head of Pterygotus, as in the former, and 

 that it also bears the reproductive organs upon its coalesced mem- 

 branous appendages. 



The branchiae would, however, be fewer in Pterygotus than in 

 Limulus, but similarly placed — resembling in this respect the im-r 

 mature condition of the latter. (See PI. II. fig. 4.) 



^ United States Exploring Expedition, 1852: Crustacea, vol. xiii. p. 21. 

 t "Lectures on the Crustacea," Medical Times and Gra.ette, 1857, p. 507. 

 VOL. XXIII. PART I. D 



