Studies on the Development of Larvai Nephridia. 499 



cells composing the rudiment of the uephridium. The exaet steps 

 by which the organ composed of a few cells is transformed 

 into the niany celled one of the fully grown larva are somewhat 

 difficult to follow, but it is doubtless from the rapid division of 

 thesc cells coinposiug the nephridial rudinieut, since their nuclei 

 rapidly decrease in size as the nephridia develop. 



The solenocyte tubes form late, and apparently very shortly 

 beforc the nephridiiira becomes functional. The externa! openings 

 of the nephridia on the ectodermal surface are remarkably small. 

 I have never been able to observe them, either in sections or in 

 whole preparations, although the spot where this opening is situated 

 is readily distinguishable on the surface of the larva frora a coUec- 

 tion of pigment about it. 



In the larva of Ph. hippocrejna the nephridial pit is much 

 deeper and better marked than in the Sicilian larva (fig. 46 and com- 

 pare figs. 38 and 37). In Ph. Sabatieri, Roule has denied strongly 

 the existence of this structure, but De Selys Longchamps (25) has 

 since found it well marked in this species. He has also found evidences 

 of the pit in the small and peculiar larva of Ph. Mälleri, where 

 it develops in a manner similar to the way its develops in other larvae. 

 Masterman has also found it in the larva in which he formerly 

 denied its existence. There is no doubt therefore tbat in all Phoronid 

 larvae it is present in the early stages. The merit of its re- 

 discovery since the time of Caldwell is due to Ikeua, who was 

 also the first to show that the nephridia arose in connection with it. 



The exact morphological siguificance of the nephridial pit is not 

 obvious, as there is no similar structure in anv of the animals to 

 which Phormiis may be related, to be comi)ared with it. 



Schultz (23) regarded it as the rudiment of the ventral pouch 

 or metasome, which of course appears much later, as Ikeda has 

 pointed out. It has several times been mistaken for the formation 



I of a proctodaeum, and this mistake is the more readily made from 



II its close relation to the rectum or gut. It is worthy of note that 

 > j in the Trochophore larva the gut opens on the exterior at the ventral 



i end of the larva in a large and couspicuous proctodaeum of ectodermal 

 i' origin, into this proctodaeum open on either side the head-kidneys 

 ' some distance from its external orifice ^. Now the relationship of 



^ E. B. Wilson was the first to call attention to this condition in 

 the Trochophore oi Hydroidcs. I have confirmed this on good number of species, as 

 P'iinatoceros, Eupomatus, and it is probal)ly the case in all typical Trochophores. 



