62 THE VOYAGE OF H.M.S. CHALLENGER. 



to me most probable, would go a long way to explain the liigli degree of sensibility of 

 every portion of the Nemertean body-wall. 



MUSCULAR SYSTEM AND CONNECTIVE TISSUE (GELATINOUS TISSUE, 



BASEMENT MEMBRANE, &c.). 



In describing in the foregoing paragraphs the integument and its varied constituents, 

 glands, .sense-cells, ciliated cells, &c., the tacit assumption has been made that the structures 

 there described might be looked upon as so many derivatives of the epiblast. Although 

 reliable embryological data are as yet very scanty, my own experience on this head 

 (XI V, XV) appeared to me to afford justification for this assumption. However, I agree 

 that the question, whether the thin layers of longitudinal or circular fibres, that, more 

 especially in Eupolia and Cerehrattdus comtgatus (PL Vll. figs. 5, 9 ; PI. XIII. fig. 6), 

 form so intrinsic and conspicuous a part of the integument, are also epiblastic derivatives, 

 or whether they are due to mesoblastic elements, is ojjen to dispute, and cannot be solved 

 for the present on any other than the a prioi'i arguments just alluded to. Hence, if I 

 look upon the tissues that are treated of in the present section as essentially meso- 

 blastic structures, I wish it to be well understood that this distinction may after all not 

 be a final one. 



I have purposely omitted discussing the basement membrane of the integument under 

 the head of the integument, because it appears to find its more natural place amongst 

 what we are now going to describe : the tissues between the outer cell laj^ers and the 

 intestinal epithelium, i.e., the muscular body- wall and the connective tissue (better, 

 gelatinous tissue, " Fiill-Gewebe "). The latter is not only present in the space between 

 the body-wall and the intestine (so far as it is not encroached upon by the generative, 

 blood-vascular, or nephridial systems), but also between the individual muscle-bundles, 

 when these are not very closely applied against each other, and outside of these, between 

 the muscles and the integument, as the so-caUed basement membrane above mentioned. 



The question as to the exact nature of this tissue is, in my opinion, a very important 

 one. It represents the tissue which in Ccelenterata fills the space between epiblast 

 and hypoblast, the "jelly" of Medusae and Ctenophora, with its nudtifarious inclusions 

 of muscular, fibrous, and eventually nervous nature. This jelly is the more im- 

 portant since its distribution, in the way above defined furnishes a strong argument for 

 the view, also held by me, that the Nemertea are devoid of a body-cavity comparable to 

 that of Arthropods, Annelids, and of Vertebrates. The only body-cavity proper to the 

 Nemertea is the modified segmentation cavity, the archicoelome, as I have elsewhere pro- 

 posed to call it (XIII, XIV). Of the cavities of the generative sacs and of the nephridia 

 mention wiU be made in the respective paragraphs. 



