THE EEPBODTJCTION OF ANNELIDS. 373 



the same process apparently begins again. There is thus an important 

 difference between the primary formed and the secondary zooids, which 

 Agassiz does not refer to, viz., that the former are more particularly 

 the result of fission, while the latter are the result of both gemma- 

 tion and fission. The former would come under Professor Huxley's 

 first category, the latter under his third. We suspect in the pro- 

 liferous development of the higher Annelids such will always be 

 the case, and that there will always be a combination of two of these 

 categories in their development. 



It is interesting to observe that the earlier stages of this Autolytus 

 closely resemble those of a Planarian worm. So remarkably is this the 

 case, that this stage might well be called the Planarian stage. Then 

 in their onward development they resemble some of the genera of 

 Annelids that we have long been in the habit of considering adult 

 forms, such as Leucodore and others — this state reminds A. Agassiz 

 of the Nematoid worms, but this resemblance is not sufficiently evi- 

 dent to us — in their last stage they have become genuine Annelids. 

 Hence it follows that for the future one must be very cautious in 

 defining new species and genera of Annelids, unless their life history 

 is known ; otherwise the parent stocks and the sexes of the one species 

 might be referred to three very different species not to say genera. 



This paper of Mr. A. Agassiz is illustrated with three excellent 

 plates, which give full details of the development and appearance 

 of the parent stock in Autolytus cornutus and of its male and 

 female zooids. It proves that the son is following well and worthily 

 in his father's footsteps, and we trust we may yet have to notice 

 many papers on this confessedly difficult group of animals from his 

 pen, even though we cannot expect them all to be as startling in 

 their details as this present one. 



XXXII. — Heebeet Spencee's Biology. 



The Peinciples of Biology. Paet IV. Moephological De- 

 velopment. Caps. ii. et iii. The Moephological Composition 

 of Plants. By Herbert Spencer. London : 1865. 



Me. Heebeet Spencee, as is known to most of our readers, is 

 engaged in the publication of a system of Philosophy. Several sec- 

 tions of this great undertaking have already been issued, but oiur 



