86 MEMOIRS NATIONAL ACADEMY OF SCIENCES, VOL X, NO. 4. 



Species A. — Species A. (tig-. 34) is somewhat smaller than Species B. and its average length is 

 1.03 mm. The trunk is quite stout, the intestine is short, and the posterior end of the stomach 

 readies as far as two-thirds of the length of the trunk cavity. When about ready to metamor- 

 phose, this larva usually has 18 larval tentacles and an equal number of young- adult tentacles. 

 The adult tentacles do not usually appear until the larva has is larval tentacles (its full number) 

 and they arise as thickenings on the under side of the liases of the larval tentacles. In this 

 respect the larva resembles one of the actinotrocha? which Ikeda (9) has described. The blood 

 corpuscles are found in two masses usually applied to the ventro-lateral surface of the stomach, 

 and they make their appearance in the larva with 12 or 14 tentacles. A pair of muscles which 

 Ikeda has been the first to describe, and which he has called " retractor muscles," are always 

 present; although they have not been made out in younger larvse than those with 10 tentacles. 

 This species is without the so-called "stomach diverticula." Pigment cells are found rather 

 irregularly scattered on the wall of the body cavity. There are definite aggregations of these at 

 the bases of the tentacles, and a few pigment cells are seen on the surface of the blood corpuscle 

 masses. Usually there are quite a number in the wall of the posterior portion of the trunk. 



This Actinotrocha is not as active as Species IS. and it docs not, as a rule, turn up its preoral 

 hood when irritated. Its metamorphosis usually takes place quickly, fifteen or twenty minutes 

 being- required for its completion. Actinotrocha Species A. is, no doubt, the actinotrocha of 

 P. architecta. 



Species Ii. {fig. 35). — This Actinotrocha is larger than Species A., and when about ready 

 to metamorphose it has an average length of 1.22 mm., and has at least 26 tentacles. (Wilson 

 (24) figures the Actinotrocha Species IS., ready to metamorphose, with 22 tentacles.) The differ- 

 ence in appearance between this larva and Species A. is rather striking. Beside being- somewhat 

 longer, it is slightly narrower in the collar region and decidedly so in the trunk region, which 

 gives it a much more graceful appearance than that of Species A. The intestine is quite long, 

 extending throughout the posterior two-thirds of the trunk cavity. 



M. Longchamps has kindly pointed out to me that the "adult tentacles appear bilaterally, 

 the mid-ventral line being, at first, free of the buds." They do not arise, however, as thicken- 

 ings on the under side of the bases of the larval tentacles as in Species A. They have their 

 origin at the base of the larval tentacles, but they are separate from them, and they appear first 

 in the larva with 24 tentacles. 



This Actinotrocha differs in three important respects from Actinotrocha Species A. In the 

 first place it has its blood corpuscles aggregated into four masses, two of which are usually in 

 the same position as the pair in the smaller species. The other two, however, are found, as a 

 rule, more anteriorly in the collar cavity, and are applied to the dorso-lateral walls of the stomach. 

 The posterior pair lying on the ventro-lateral sides of the stomach make their appearance during 

 the 18 or 20 tentacle stage, hut the other pair do not appear until about the 22-tentacle stage. 

 This larva also has retractors extending from the ganglion to the region of the first and second 

 pair of tentacles. 



A second point of difference is the fact that Actinotrocha Species B. possesses a pair of 

 diverticula at the anterior end of the stomach. These are present as early as the 22-tentacle 

 stage. This larva can further be distinguished from the other species by the fact that there 

 is found in the older larva- a sensory papilla on the mid-dorsal surface of the preoral lobe. 



Actinotrocha Species IS. is much more active when irritated than the other species. The 

 least irritation causes it to turn up its hood and to assume attitudes like those figured by 

 Masterman (15). In fact, judging by the figures and text of Masterman's paper, it seems that 

 there is considerable similarity between this larva and the one he has described. The two larvse 

 are very much alike in shape and both have the lateral stomach diverticula, but the form that 

 Masterman describes has only two masses of Mood corpuscles. The two species are not identical, 

 nor is Actinotrocha Species IS identical with Actinotrocha ivanchiata from the North Sea, for, as 

 Longchamps has pointed out to us, the latter has but two blood corpuscle masses. Long-champs 

 has informed us that in Actinotrocha Species IS. the adult tentacles make their appearance in the 



