NEW ENGLISH GENUS OF AQUATIC OLIGOOH^TA. 171 



walls. He surmises — and this^ I think, is from an a priori 

 view probably correct — that the wall arises from the wander- 

 ing cells of the mesoblast, and the blood-vessels are probably- 

 formed, as in the Vertebrata, as spaces amongst the mesoblast- 

 cells, which then give rise to the wall of vessel. 



The suggestion that they represent blastocosl seems as little 

 true as that the coelom does so ; of course, both occupy the 

 position of the blastocoel. 



From the point of view of comparative anatomy, too, the 

 existence of a sinus does not seem an archaic one, for it is not 

 in simple or '' primitive " Polychsetes, for example, that this 

 exists, but in the more evidently modified tubicolous forms 

 belonging to the families Serpulacea, Terebellacea, Tel- 

 thusida, Chlorhsemida, and others. I say these are not 

 primitive forms, and among many reasons for this view I 

 may point to the character of the excretory system in these 

 families ; in place of the metameric repetition of simple and 

 similarly formed nephridia, we find suppression of some and 

 enlargement of others ; or a differentiation, again, to form 

 excretory organs and genital ducts. A second character upon 

 which I should rely in support of my suggestion is in the 

 accompanying disappearance, or at least extreme reduction, of 

 the septa. 



On the other hand, in such simple forms as Polygordius 

 and Protodrilus — whether this simplicity be primitive or 

 due to degeneration is a matter for discussion — we find no 

 perienteric sinus, and indeed but little in the way of an 

 enteric vascular system. In the more generalised Nereis 

 and its allies, which may perhaps more closely represent 

 a primitive Polychsete — not an ancestor of this group, but 

 an early representative of the group, with all the charac- 

 teristic features well marked — an elaborate plexus around the 

 gut exists. 



In fact, the purpose of a vascular system being to distribute 

 nutriment, &c., its usefulness would evidently be increased if 

 the contained fluid were to take a definite course. This would 

 not be the case in a great sinus, though perhaps interchange of 



