GENERAL CONSIDERATIONS. 79 



section passes over into the peduncle of the adult. Consequently, 

 the cavity which is surrounded by the posteriorly projecting mantle- 

 fold of the Brachiopodan larva is to be homologised only with the 

 •cavity within the so-called sucker of the Bryozoan larva. In the 

 Bryozoa, also, that point of the sucker at which fixation first takes 

 place forms a conical invagination in the main mass of the sucker (cf. 

 the larva of Bugula, Fig. 9 B, p. 26). This conical portion would 

 thus directly correspond to the pedal region of the Brachiopodan 

 larva. This whole structure is evidently the equivalent of the 

 invaginated ventral tube of the Actinotrocha. The aperture of this 

 tube in the larva of Phoronis would be comparable to the aperture 

 ■of the mantle-cavity in the Arglope larva. We can therefore without 

 ■difficulty compare the Brachiopodan larva with an Actinotrocha in 

 the later stages, in which the above tube-like invagination has 

 already formed. The principal distinction between the two is the 

 •complete absence of the anal region in the Brachiopodan larva. 



Here also metamorphosis commences (as in Phoronis and the 

 Bryozoa) by the evagination of the formerly inturned part of the 

 body through which fixation takes place. 



We thus find that a comparison of larval forms leads us to regard 

 the Brachiopoda as nearly related to Phoronis and to the Bryozoa, 

 and that a comparison of the anatomy of the adult Brachiopoda 

 with that of adults belonging to the two other groups gives a general 

 confirmation to this view. The agreement between these groups is 

 specially clear if we compare the tentacle -bearing lophophore in 

 Phoronis, the Phylactolaemata, and the Brachiopoda. In the latter 

 also, the lophophore is originally horseshoe -shaped, there is an 

 integumental fold known as the epistome (Fig. 41, ep) above the 

 mouth (brachial fold), and a lophophore -cavity not in any way 

 communicating (?) with the rest of the body-cavity. We shall have 

 to regard the so-called small brachial sinus (Fig. 41, a) of the 

 Brachiopoda as the equivalent of the lophophore-cavity proper of 

 the Phylactolaemata (circular canal of the Gymnolaemata) while 

 the large brachial sinus (b) perhaps corresponds to the epistomal 

 cavity.* Certain differences between the cavity in the lophophore 



* In comparing the large brachial sinus with the epistomal cavity of the 

 Phylactolaemata, we shall have to hear in mind that the large arm-sinus, as 

 has recently been pointed out by Blochmann in Crania, is a paired structure, 

 continuous in the median plane above the oesophagus. The median section 

 (Fig. 41 B) represents the condition of the spiral arms laterally to the median 

 plane. If we bear in mind the above-mentioned reservations, however, we shall 

 De able to recognise in this diagram the agreement of the large brachial sinus 

 with the epistomal cavity. 



