INTESTINAL RESPIRATION IN ANNELIDS. 769 



anterior. The best that can happen, therefore, is that the ventral vessel should simply 

 lose its original contractility altogether. 



This it does ; muscle cells are indeed abundantly present, as has been shown, among 

 others, by Vejdovsky (55), Schneider (42), and Sterling (52), for both lower and 

 higher forms ; but, although isolated statements to the contrary have been made, it will 

 be admitted that the ventral vessel is in general non-contractile throughout the group. ■^'" 



The Eayiiest Differentiations are in the Middle Line. — Can we assign any reason why 

 the specialised longitudinal tracts in the intestinal network, which come to form the 

 dorsal and ventral vessels, should appear in the median line ? 



The lateral bending movements of a worm-like animal must cause temporary 

 obliteration of the spaces on the sides of the alimentary tube on the convexity of the 

 curve by the stretching out of the wall, and on the concavity by the squeezing 

 together of the wall in which the spaces lie. The only regions unaffected by these 

 deformations are the mid-dorsal and mid-ventral lines ; hence along these lines clear 

 channels for the flow of fluid might be formed, which would not be impeded in con- 

 sequence of lateral flexions of the animal's body. 



It is, of course, the case that the chief vessels in Nemertines are laterally situated ; but 

 they are not in the same close connection with the alimentary canal as in the forms now 

 under consideration, and it is at least possible that the vascular system did not arise in 

 the same way in that group. 



The Supraintestinal Vessel. 



The supraintestinal vessel is a differentiated portion of the alimentary network 

 which exists only in certain of the anterior (oesophageal) segments of the body in the 



* Beddard (3) states that the ventral vessel is contractile in Phreorydes. Harrington (a2md Lang, 29) makes the 

 same statement with regard to earthworms, though the pulsations " are never so well marked as in the dorsal " vessel. 

 Schneider (42), in saying " Somit sind beim Regenvvurra alle Getasse mit Ausnahme der kleineren Venen und der 

 Kapillaren kontraktil," is apparently arguing merely from the presence of muscular fibrils, and therefore begs the 

 question. Sterling (52), while admitting the ventral vessel in earthworms to be " sehr wenig kontraktil," adds 

 "auch das Studium ganz junger durchsichtiger Embryonen erlaubte mir nicht, die Kontraktilitat deutlich zu 

 beobachten." FucHS (19, p. 477) does not allow the ventral vessel among those portions of the vascular system of 

 Annelids that may show contractility ; he states, however (ibid., p. 460), that according to CLAPARfeoE it is contractile 

 in Haplotaxis, and adds, " Das ist die einzige mir bekannt gewordene positive Angabe von einem kontraktilen Vas 

 ventrale." Vejdovsky (55) takes up the same position as Schneider ; thus (loc. cit., p. 140) he says, " Dass die Intestinal- 

 gefasse gewiss kontraktil sind beweist ihre muskulose Umhiillung, namentlich der in das Riickengefass ausmlindende 

 Abschnitt" ; and again (loc. cit., p. 157), in a statement which includes the ventral vessel, which is our present concern, 

 " Samtliche Bestandteile des GefasssysLems sind mit Muskelfasern versehen, und somit miissen alle kontraktil sein, 

 mag man sie auch als nicht kontraktil bezeichnen." The argument seems to me to be defective, and much as if one 

 should argue that a man must be able to move his ears because he possesses a series of muscles attached thereto. 

 Vejdovsky expresses himself more cautiously in another place (loc. cit., p. 147), where he says, " Es sind also dem Baue 

 nach alle Gefasse kontraktil, oder besser, alle bestehen aus Muskelzellen." 



Taking the last statement as correct, is it not possible to apply the principle of change of function ? The vascular 

 system is developing, throwing out extensions ; the material at hand for building it is muscle cells. These are not 

 required to be contractile throughout ; hence, though still recognisable as muscle cells, they become, in certain parts 

 of the system, merely passive building materials. They may even lose their distinctive histological characters : cf. 

 Vejdovsky, in a sentence which occurs immediately before the one last quoted, " Schliesslich sind die Zellea, aus 

 welchen die feinsten Oapillaren bestehen, in die Kategorie der Muskelzellen zu stellen, wenn auch hier keine fibrillare 

 Struktur uachweisbar ist." 



At any rate, the question of contractility is rather one for actual observation than for decision by inference from 

 histological structure. 



