CASES SIMULATING ALTERNATION OF GENERATIONS. 225 



essence of organic life consists. One does not see wliy, for 

 instance, the exuviation of sheets of cuticle after scarlet 

 fever is not as much entitled to be called ** a concentric 

 fission" as the ecdysis of an insect in the course of its me- 

 tamorphosis. 



§ 5. Another class of cases which in some degree simi- 

 late those of alternation consist in the formation of sper- 

 matophores and nidimentary etivelopes. These structures, 

 which are sometime elaborately organized, have been met 

 with in Cephalopoda, Gasteropoda, Insecta, Crustacea, 

 Annelida, Planaria, and perhaps other Invertebrata. In 

 the Cephalopoda the spermatic corpuscules, when generated 

 in tlie usual way, become enclosed in peculiar cases or re- 

 ceptacles, which in the case of the Lolligo have been long 

 known to naturalists under the name of " filaments of 

 Needham." They are of a very complex structure, but 

 their motile powers, chiefly shown in the ejection of their 

 contents, may probably be accounted for on physical prin- 

 ciples.* In the Argonauta, in which the spcrmatophore is 

 a simple sac, its active functions are discharged by the pe- 

 cidiarly modified tentacle (Hectocotiflus) in the cavity of 

 which it is lodged previously to the member being thrown 

 off from the body of the Cephalopod.*|" Sperm atophores of 



• 



* In Nautilus the spermatopliores after ejaculation appear to be coiled 

 up witliin the funnel iuto peculiar laminated vesicles. When uncoiled 

 they appear as attenuated filaments about half a line in diameter, but of 

 great length — e.g., 27 to 34 centimetres in Nautilus, 3 feet in Octopus 

 Carena (Leuckart), 8 centimetres in Octopus vulgaris (M. Edwards). 

 Hoeven, in Ann. Nat. Hist., 2d Ser., XIX., 72. 



t Annals of Nat. History, 2d Ser., IX., p. 492. Ann. des ScienceB 

 Nat., i. XVI., No. 3. From the observations of Steenstrup it seems 

 probable that in all male Cephalopoda one of the eight shorter arms i.s 

 different one side of the body from its fellow on the opposite side, and ia 

 developed for a certain extent of its length in a manner more or less 

 analogous to that of the Hectocotylus-arm of the Argonauta. The par- 

 ticular arm and the side of the body afibcted vary in diflcrent species. 

 Anuals of Nat. Hist., 2d Ser., XX., 81. 



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