604 MONTGOMERY— MORPHOLOGY OF THE [April 24, 



lack of ciliated epithelia is a histological characteristic of the groups 

 that possess them. 



(b) Mesectohlastic Organs. 

 Here are to placed the following structures : 



5. Scattered excretory cells, such as connective tissue elements 

 of the Mollusca, and possibly the bacterioidic cells of the Oligochseta. 



6. Closed vesicles, the kidney sacs of Tunicata, and possibly 

 the nephrocysts of nudibranch Mollusca. These seem to act as 

 centers of accumulation of waste substances. 



7. Tubes communicating with the exterior, the inner ends blind 

 and terminating with a flame cell or solenocytes. In all probability 

 the larval nephridia (head kidneys) of Oligochseta and Polychseta 

 belong here (in the latter sometimes a portion of the duct is strictly 

 ectoblastic) ; possibly the nephridia of the Dinophilea fall also into 

 this category, but nothing is known as yet of their development. In 

 their structure these are very similar to the organs of type 3, the 

 protonephridia in the restricted sense, the only difference being that 

 the one come directly from the ectoblast, the others from the 

 mesectoblast. 



(c) Organs Wholly or Partially Mesentoblastic. 

 These represent the more specialized kinds of excretory organs, 

 correspond in part to the metanephridia of Hatschek, and may be 

 subdivided into the following main types : 



8. An ectoblastic invagination joining directly (without partici- 

 pation of retroperitoneal mesentoblast) with a coelomostome (peri- 

 toneal funnel), the involved portion of the coelom not exclusively 

 excretory. Examples are the adult nephridia of Phoronis, and 

 the head and collar pores of the Enteropneusta ; homologous with 

 these is the stone canal of the Echinodermata. The present evi- 

 dence does not allow us to decide whether the segmental organs of 

 the Sipunculida, Ectoprocta, Brachiopoda, Echiurids and Myzo- 

 stomes belong with this type or with type 11. 



9. An ectoblastic invagination joining directly (without partici- 

 pation of retroperitoneal mesentoblast) with a reduced coelomic 

 sac, the latter being an exclusively excretory end sac. There are 



