MEMOIES OF THE NATIONAL ACADEMY OF SCIENCES. 83 



idea thai a stomodajum 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 "(low awards (ill it is lost on the surface of the tentacles." Such a ridge is not present in 

 the larva of Pharonis architecta and there i- no connection between the ciliated tract alone- the 

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



At this stage (tie'. 24) a definite intestinal canal i- seen which, however, does not open yet to 

 the 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: "1*11 anus et rectum se faconnent, aux depens de I'ectoderm, sur I'extremite' 

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

 "primitive streak." 



As vet the anal papilla is not at all definite, but the ciliated hand along which the larval 

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

 thickening of the ectoderm. 



The mesoderm cells which in tie-. 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 

 (fie-. 24). If the ventral surface of the larva is examined, while the larva is alive, it will he 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 (fig. 25). 



The whole body cavity hack 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, yef there are indications that such a 

 lining i- 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 be 

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

 preoral lobe. 



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

 are dealing certain mesodermal cells to which he has given the name "conjunctivo-musculaircs 

 elements." These he represents as spindle-shaped cells terminated by lone- 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 Phoronis architecta bears a close 

 resemblance to that of Phoronis sdbatieri 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 tin- lobe-collar 

 septum, yel such a septum unquestionably exists in the larva of Plwronis architecta. Ikeda 

 (9) has shown that it i-- 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 on moins complete s'etablissait, entre ces 

 deux regiones, elle ne seriat en tout cas one secondaire, et la cloison s'edifierait 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 anil the collar i- often considerably thinner than that between the collar and 

 the trunk. If its origin had nol been followed from the earliest stages by mean- of section- 

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

 consider it a primary and constant organ of the larva. 



