PHORONIS ARCHITECTA— BROOKS AND COWLES. 109 



the wall of the collar and trunk, lying between the longitudinal fibres and the ectoderm. There 

 is also a covering- of muscle cells on the ventral pouch and on the wall of the dorsal blood vessel. 



The nephridia have much the same structure as those of Amphioanus, as described by Good- 

 rich. In one of the Actinotrocha from Beaufort Harbor the nephridial canal branches, but in the 

 other it does not. Nephridial funnels do not exist, but the ends of the canals open into tubular 

 cells, and the lumen of each cell contains a flagellum. The nephridial canals open to the exterior, 

 at the sides of the orifice of the ventral pouch. The nephridia, which Masterman describes for 

 the preoral hood and trunk, are not present in any Actinotroch.se that we have examined. 



The bloodvessels of the Actinotrocha are formed from the splanchnic mesodermal lining and 

 they inclose part of the blastocoele. There isa dorsal blood vessel opening ('() anteriorly into the 

 space between the stomach wall and the splanchnic lining. At the posterior end of the stomach, 

 where the dorsal vessel ends, there are caecal vessels formed as evaginations of the mesodermal 

 lining of the stomach. The dorsal vessel becomes the afferent vessel of the adult, while the 

 efferent vessel does not arise until just after metamorphosis. The collar cavity, which isa part 

 of the blastocoele, becomes the ring vessels and tentacular vessels of the adult. There is no 

 connection between the dorsal blood vessel and Masterman's "subneural sinus." 



The blood corpuscles arise from the somatic mesodermal lining of the ventro-lateral collar 

 wall just in front of the septum. The\ make their appearance as two masses of cells bilaterally 

 placed, one on each side of the median ventral line, and as they develop they migrate across the 

 collar cavity and become applied to the naked walls of the stomach. 



The rudiment of the supraseptal or collar cavity of the adult makes its appearance in about 

 the same region as do the blood corpuscles but a little later in the life history of the Actinotrocha. 



During metamorphosis the following organs are lost: The preoral lobe, the ganglion, and 

 the larval tentacles. The ectodermal wall of the collar cavity, the stomach diverticula, the 

 digestive areas, and the perianal ciliated ring are not destroyed, but they lose their identity. 

 The subepidermal nervous layer of the trunk and ventral pouch becomes part of the same 

 tissue in the adult, but the larger part of this tissue, as well as the lateral nerve, the ganglion, 

 and the nerves to the lophophoral organs are new formations. 



All of the nervous structures of the collar and trunk are lost during metamorphosis, except 

 the collar trunk nerve ring, which persists as the nerve ring of the adult. 



The ventral mesentery becomes the oesophageal and rectal mesenteries of the adult, and the 

 cavities of the trunk and ventral pouch are transformed into the infraseptal cavity. 



At least the greater part of the nephridia is lost during metamorphosis. 



The lophophoral organs arise late in the adult life and are present only in individuals which 

 are with testes and without ovaries. They probably serve as seminal receptacles. 



The vascular system of the adult consists of an efferent and afferent vessel, which are 

 continuous posteriorly by means of a sinus around the loop of the alimentary canal; of caeca! 

 vessels as outgrowths from the afferent vessel and the blood sinus; of a distributing and recip- 

 ient ring vessel, the former opening into the afferent vessel and the latter into the efferent vessel; 

 and of tentacular vessels, each of which divides into two short branches, one opening into the 

 distributing vessel and the other into the recipient vessel. 



There is a ciliated ridge extending along part of the inner wall of the alimentary canal. 



The nervous system of the adult is to a great extent subepidermal. There is a nerve with 

 a nucleated sheath extending along the left side of the animal. Anteriorly it bends around the 

 anal papilla and continues as a short nerve on the right side. There is a ganglion between the 

 mouth and anus. A nerve ring extends around the base of the lophophore and it gives off 

 nerves to the lophophoral organs. There is nervous tissue in the walls of the tentacles. 



The excretory organs are paired and each nephridium consists of a tube bent upon itself. 

 One end opens to the exterior, while the other is continued into two funnels, one communicating 

 with the rectal and the other with the lateral body cavity. 



The reproductive organs arise from the lining of the caeca] blood vessels, and the male organs 

 develop at a different time from those of the female. 



.Johns Hopkins University, March. 190^. 



