THE ANATOMY OF PHORONIS AUSTRALIS. 149 
closure lying on the ventral surface. In the Polyzoa the 
blastopore closes, and the anus later on appears in the position 
occupied by it. However, in the Mollusca the same variety 
in the behaviour of the blastopore is known, e.g. Limnezeus, 
Paludina, &c., so that no great value need be attached to this 
point of difference. 
The mesoblast arises in a very different manner in the 
two groups. In Polyzoa it arises from large pole-cells, which 
lie on each side of the blastopore; but in Phoronis, Caldwell 
has described a double origin for this layer: (a) by cells 
budded off from the outer surface of a pair of pouches of the 
archenteron ; (b) by invaginated cells posteriorly. 
c. Relations of Phoronis to Sipunculids. 
There are more points of undeniable resemblance between 
these, than between Phoronis and Polyzoa, both in adult and in 
larval anatomy. This is, however, largely due to the uncertainty 
existing as to the true orientation of the Polyzoa. 
The mouth and anus are closely approximated ; the space 
between them being dorsal. The ‘ epistome” (i. e. prosto- 
mium) appears to have no representative in adult Sipunculids ; 
and if, during the metamorphosis of Actinotrocha, the dege- 
neration which already takes place in this part of the body 
were carried a step farther, we should lose all trace of the 
prostomium. As it is, the remnant is very small, and were it 
not for Caldwell’s precise statements on the point, we might 
feel inclined to doubt whether it really be pre-oral, or whether 
it is merely a dorsal fold which remains between the larval 
tentacles and the permanent ones. 
The tentacles of Sipunculus (and presumably of other 
genera) are stated by Hatschek (13, p. 55) to be processes of 
the lips, which appear first at the side of the mouth, and to 
have nothing to do with the ciliated band of the larva. In 
Phoronis the adult tentacles are formed behind the circlet 
of larval tentacles, which are eaten. During the larval life 
'3« Arb. Zool. Inst. Wien,’ 1884, 
