CEANGON VULGARIS. 19 



occurs in the Cephalopoda according to the observations of 

 S. Watase ('88) . Studying both Loligo pealii and an un- 

 known West Indian species of cephalopod this author con- 

 cludes (p. -178) "As to the origin of the digestive tract 

 with its appendages, I found it to be entirely formed by 

 the ectodermic invaginations, that is by the prolongations 

 of the proctodeum and the stomodeum." Of course this 

 is but analogy and the cephalopod differs from Crangon in 

 having, according to Watase, none of its diverticula derived 

 from the morphological endoderm. 



To account for the strange condition which he describes 

 Watase has recurrence to the modifying influence of the im- 

 mense food yolk. "In the Cephalopod the endoderm be- 

 comes the yolk digesting membrane, and before it can 

 dispense with this function, the prolongations of the fore 

 and hind guts complete the digestive tube and exclude the 

 endoderm, which itself becomes absorbed later." Though 

 there is a tendency to a belief that food yolk is called upon 

 to explain more than it ought to explain, I feel confident 

 that in both the cases of Crangon and the Cephalopods it 

 affords a valid reason for the conditions described. 



The change in the position of the anus is interesting. 

 At first it is distinctly dorsal, a condition not uncommon 

 in the adults of many annelids, while by the outgrowth of 

 the upper margin of the opening it soon becomes terminal 

 and at last, long before hatching, it assumes its crusta- 

 cean position on the ventral surface. 



"DORSAL ORGAN." 



In figures 37 and 39 are shown two sections of the prob- 

 lematical structure which I have already referred to as the 

 "dorsal organ". I have nothing to add to the account 

 which I formerly gave of it in this Bulletin (Vol. xvm, 

 p. 149, pi. n, f. 19). It is solely ectodermal, and so far as 



