794 RECORD OF CURRENT RESEARCHES RELATING TO 



their primary purpose an increase of the internal surface of the 

 enteron. The differences lie in the degree to which the folds or 

 septa are developed. In the Actinias there are a number of them, 

 and they are arranged radially around the enteric axis ; in the 

 Chsetognatha there are only two, and these two are arranged in a 

 bilaterally symmetrical fashion, while, instead of continuing to pro- 

 ject into the enteron by their free edges, they become shut off from 

 it and converted into two closed sacs. 



Passing to the question of their functions, we find both in the 

 Actiniae and in the Chaatognatha that the generative organs and the 

 musculature of the body are developed from the endoblast ; the 

 development of the nervous system has, moreover, probably, though 

 not certainly, exactly the same history in both these groups. 



Striking as these resemblances are, they are not sufficient to justify 

 us in affirming any closer relation of these two divisions than is 

 as yet allowed ; all that we see is that there is in the develoj)ment of 

 organisms certain fundamental laws which are obeyed by different 

 animals ; and the work on which Hertwig is here engaged is the 

 study of the laws of the formation of organs and of tissues. 



The ChcBtognatJia and the other Worins. — The Chaetognatha seem, 

 as other naturalists have already noted, to be most nearly allied to 

 the Nematoids and Annelids ; with the former the most important 

 resemblances lie in the fact that in many of them (Gordiacea) the 

 enteric canal is attached to the dermo-muscular tube by a dorsal and 

 ventral mesentery : together with other points, we have to note that 

 in other Nematoids the muscles form plates which are set perpen- 

 dicularly to the surface of the body, and are made up of parallel 

 fibrils. The relations of Sagitta to the Annelids is still more 

 remarkable ; if we make a transverse section through a Sagitta and 

 compare it with a somewhat old larval stage of Polygordius, we find 

 that in both cases the enteron is invested in a fibrous enteric layer, 

 and is attached by mesenteries, which completely divide the coelom 

 into a right and left half. In both cases there are four bands of 

 longitudinal muscular fibres, the cells of which are derived from the 

 ccelomatic surface. Points of resemblance to Spadella cephaloptera 

 are to be observed in the development of transverse muscular fibres 

 on the inner side of the ventral muscular bands and in the minute 

 structure of the fibres. The two transverse septa of the Chaetognatha 

 are comparable to the numerous transverse septa in Annelids, while, 

 lastly, in both groups the generative products are derived from cells 

 of the parietal layer of the mesoderm. 



Are these resemblances analogical or homological ? To answer 

 this question we must first answer these others. Has the mesoblast 

 been formed by the development of folds or by the differentiation of 

 cells ? Is the coelom an enterocoele or a schizocoele ? These are the 

 questions which the author is anxious to bring to the fore, and, until 

 they are answered, we cannot speak confidently as to either the 

 systematic position of the Cbfetognatha, or as to that of other phyla 

 and divisions. 



We must deal briefly with the details of the anatomy and histology 



