150 WILLIAM BLAXLAND BENHAM. 
the adult tentacles appear first at the sides, new ones being 
added dorsally and ventrally. 
As in Polyzoa, there is no transverse septum in Sipunculids 
separating the tentacular cavities from the visceral portion of 
celom. The tentacles and anterior region of the body can 
be retracted in Sipunculus, as in Polyzoa. 
In the possession of a definite vascular system,! Sipunculus 
agrees with Phoronis, and in many Sipunculids there is a more or 
less definite circular vessel, with czcal vessels passing from it 
to the tentacles, although more usually the vessels form a 
plexus in the tentacles. In young Phascolosoma there 
are branching cecal processes given off from the longitudinal 
vessel, similar to those in Phoronis. 
The nervous system of Sipunculids is much more deeply 
seated than in Phoronis, consisting of a definite ganglion, 
with a longitudinal cord passing along the ventral surface. 
The region from which the sexual cells are developed in 
Sipunculids is a matter of some dispute. According to Koren 
and Daniellsen (14) they are developed from the cclomic 
epithelium near the commencement of the intestine, in a 
position, that is, in which a stomach would be, and to some 
extent, therefore, agree with the position in Phoronis. 
According to Semper, however, the cells arise from the 
ceelomic epithelium covering the bases of the retractor muscles 
(ef. funicular origin in Polyzoa). The glands, moreover, are 
paired and the Sipunculids are unisexual. 
Hitherto we have scarcely more points of agreement than 
between Phoronis and Polyzoa. Butif the larva of Sipun- 
culus be compared with Actinotrocha, numerous points of resem- 
blance, some of them very important, will be observed, whereas a 
similar comparison with Polyzoa shows few, if any, such points. 
In Sipunculus the blastopore closes, as in Polyzoa. 
The mesoblast arises in the same way as in Polyzoa, and 
other groups, viz. from certain pole-cells, close to the blastopore. 
1 In Sipunculus there is a communication between the blood-vessels and the 
cecelom. 
44 «Fauna littoralis norvegie,’ 1877. 
