164 DR. COBBOLD Oi!f AKIMAL ITSTDITIDtTALITT. 



series, these temporary life-phases display every degree of dis- 

 tinctiveness from the almost imperceptible up to the separable, 

 free, individual-like being for the designation of which Prof. Huxley 

 has felicitously proposed the term " zooid." All these phases are 

 now known to be phenomena of growth, metamorphosis, and 

 gemmation, there being no such thing as "alternate generation" 

 in the truest sense of this phrase *. 



Applying these principles to the interpretation of the pheno- 

 mena of entozootic life, some very curious results appear to be 

 attainable when we come to deal with the more complicated forms. 

 Starting, however, with a species where the individual is repre- 

 sented by simple, non-metamorphosed life-phases, we necessarily 

 encounter the almost indistinctive conditions of ordinary growth. 

 Thus I select, in the first instance, the so-called Trichina spiralis, 

 whose life-phenomena (according to the synoptical method ini- 

 tiated in my work on Entozoa) may be tabulated as follows : — 



ZooiiOG-lCAl iNDiviDTJAli {Trichina spiralis). 

 a. Ovum in all stages. 

 h. Intra-uterine embryo. 



c. Free embryo or migrating larva. 



d. Resting or sexually immature larva. 



e. Expectant or sexually distinctive Trichina (often encysted). 



f. Free, sexually mature intestinal Trichina. 



Now, although the various larval stages above indicated bear a 

 general resemblance to the adult Trichina, we have, even here, 

 some faint traces of " epochs " which, were they only rather more 

 strongly pronounced, would enable us to draw lines of demarcation. 

 In some instances the life-epoch may be homologically identical 

 with a temporary bud, but it may also comprise a multitude of 

 gemmce. Each such successive life- epoch, whether distinctive or 

 indistinctive, separable or inseparable, I propose to call a liotome ; 

 and when two or more such life-divisions are recognizable, I pro- 

 pose to call them "secondary" or "tertiary" biotomes, as the 

 case may be. I would observe that the term " biotome " is not 

 designed to supersede the term "zooid," but rather to limit the 



* Since this passage was written, I have received an important communica- 

 tion from Professor Leuckart, in which he states that he has reared sexually 

 mature free Nematoids, of the genus Rhahditis, from Ascaris nigrovenosa. If 

 this be the case, we have here, for the first time, a true alternation of generation, 

 or, to say the least, a true sexual dimorphism in animals (Nachrichten von 

 der Konigl. Q-eseUschaft der Wissensch. zu Gottingen, No. 8, April 19, 1865, 

 p. 227). 



