1902. CONKLIN — EMBRYOLOGY OF A BRACIIIOI'OD. 65 



tentacles appears first and that successive pairs of tentacles are 

 added dorsally, exactly as in Actinotrocha. (4) In the meta- 

 morphosis the mantle (lophophore) is turned forward over the pre- 

 oral lobe in exactly the same way in both cases. These are 

 extremely important resemblances, and in themselves lend support 

 to the view that Phoronis and the Brachiopoda are closely related.' 



On the other hand, according to Masterman's ('97 and 1900) 

 recent work on Actinotrocha, there are certain important respects 

 in which Actinotrocha differs decidedly from the brachiopod larva : 

 (i) The ccelom consists of an anterior unpaired cavity and of two 

 pairs of cavities, one of which lies in the lophophore and the other 

 in the trunk region. The anterior unpaired cavity somewhat 

 resembles in position and method of origin the anterior portion of 

 the enterocoel of Terebratulina, but the lophophoral and trunk 

 cavities of Actinotrocha differ from the mantle and peduncular 

 coelom of Terebratulitia in that the latter are a part of the entero- 

 coel and are never completely separated from one another, whereas 

 in Actinotrocha they arise as schizocoels and are always separate. 

 (2) Actinotrocha also has rudiments, at least of a second pair of 

 nephridia. (3) It also has two endodermal outgrowths from the 

 anterior portion of the enteron, which are composed of large vacu- 

 olated cells, and are homologized by Masterman with the notochord 

 of the Hemichorda.^ 



I have had no opportunity of studying the later stages in the 

 development of the brachiopod, in which alone the two last-men- 

 tioned structures might be looked for, and cannot therefore deter- 

 mine whether there are real differences between the brachiopod and 

 Phoronis in these respects. With regard to the differences shown 

 by the coelom, one must bear in mind the fact that in the brachio- 

 pod larva the coelom almost entirely disappears, except in the man- 

 tle, and a segmentation of the coelom in later stages could not 

 therefore be observed, even if it had at one time existed in the 

 ancestors of the brachiopods. There can be no doubt however 

 that in Terebratulina the entire coelom arises from a single entero- 

 coel, in which respect there is a decided difference between the 

 brachiopod and Phoronis. The resemblances mentioned above 



1 The presence of " plasmic corpuscles " (Ideka, 1901) in the blastoccel of both 

 forms is another interesting resemblance (see p. 45). 



2 See postscript, p. 70. 



PROG. AMER. PHILOS. SOC. XLI. 168. E. PRINTED MAY 6, 1903. 



