PHORONIS ARCHITECTA— BROOKS AND COWLES. 83 



idea that a stomodseum is a pitting in of the ectoderm which finally breaks through into the 

 enteric cavity is correct, then Masterman's and Longchamps's use of the word is incorrect.) 



Masterman speaks of a "slight ridge" running around the edge of the preoral hood and 

 then "downwards till it is lost on the surface of the tentacles.*' Such a ridge is not present in 

 the larva of Phoronis wrchitecta and there is no connection between the ciliated tract along the 

 line of which the larval tentacles arise and the ciliated edge of the preoral hood. 



At this stage (tie - . 24) a definite intestinal canal is seen which, however, does not open 3*et to 

 i he exterior. The intestine, as described above, is not of ectodermal origin in the larva of Pho- 

 ronis architecta. There is no proctodeum. On this point our observations agree with those of 

 Masterman, Longchamps, and Ikeda. 



Roule (20) says: " Lin anus et rectum se faconnent, aux depens de l'ectoderm, sur I'extremite 

 posterieure du corps " (p. L02), and Caldwell (3a) derives the intestine from the remains of the 

 "primitive streak." 



As yet the anal papilla is not at all definite, but the ciliated band along which the larval 

 tentacles are to arise has now appeared. This is indicated in the sagittal section (tig. 24) by a 

 thickening of the ectoderm. 



The mesoderm cells which in fig. 21 are seen applied to the ventral ectoderm of the larva 

 have now increased considerably in number and have become arranged at quite definite intervals 

 (titr. 24). If the ventral surface of the larva is examined, while the larva is alive, it will lie seen 

 that these cells have become simple muscle cells made up of two rather delicate fibres which 

 extend from a large nucleus situated near the mid-ventral line. These fibres run parallel to one 

 another around the wall of the larva (tie-. 25). 



The whole body cavity back of the mesentery between the cavities of the collar and lobe 

 represents the larval collar cavity of the Actinotrocha, and although its somatic walls are not 

 lined by a perfectly continuous mesodermal epithelium, yet there are indications that such a 

 lining is being formed. The ventral and lateral walls of the stomach, however, are perfectly 

 free from any epithelial covering. In fact, in all the Actinotrocha' examined no mesodermal 

 epithelium covering the ventral and lateral walls of the stomach in the collar region could lie 

 found. We have never seen any sign of mesodermal sac-like formation such as occurs in the 



preoral lobe. 



Roule (20, p. 112) has described in a considerably older larva than the one with which we 

 are dealing certain mesodermal cells to which he has given the name " coniunctivo-musculaires 

 elements." These he represents as spindle-shaped cells terminated by long fibre-like prolonga- 

 tions and he has figured them as being quite numerous in the "plasma, transparent et con- 

 sistant." of the coelomic cavity. While the young larva of Phormiis architecta bears a close 

 resemblance to that of Phoronis s<ih<if;,ri described by Roule (20), yet at no time during the 

 life of the larva have we seen these cells suspended in the body cavity in such numbers as he has 

 shown. Spindle-shaped cells with long prolongations are quite numerous, but they are usually 

 found applied to the somatic walls of the larva. 



Although recent investigators have thrown some doubt on the existence of the lobe-collar 

 septum, yet such a septum unquestionably exists in the larva of Phoronis architecta. Ikeda 

 CM has shown that it is incomplete in the old actinotrocha and our observations agree with his. 

 but it is a fact, nevertheless, that the septum is continually present throughout the larval life of 

 Phoronis architecta and that it makes its appearance at a very early stage in the life history. 



Longchamps (12) says: "Si une subdivisions plus ou moins complete s'e"tablissait, entre ces 

 deux regiones, elle ne seriat en tout cas que sccondaire, et la cloison s'&lifierait aux depends de 

 mesenchyme." It is plain from what has been said that we can not agree with Longchamps 

 in his statement that the septum is secondary. It must be admitted, however, that the septum 

 between the lobe and the collar is often considerably thinner than that between the collar and 

 the trunk. If its origin had not been followed from the earliest stages by means of sections 

 in three different planes, whole mounts and live material, we should not have been inclined to 

 consider it a primary and constant organ of the larva. 



