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POLYZOA 



401 



a free swimming larva; liut the ridge of cells bearing them can he 

 retracted, and then they point inwards towards the atrial cavity. 



The mesoderm cells have meanwhile developed into short meso- 

 dermal bands on each side. According to Lebedinsky (1905) each 

 of these bauds becomes divided into three spherical somites, and in 

 each somite a cavity appears, which we may regard as coelornic in 

 character, just before the larva emerges from the parent vestibule. 

 The first pair of somites become connected with the atrial cavity by 

 long ciliated ducts which represent the excretory organs ; the second 

 pair of somites give rise to the ovaries, and the third to the testes 

 (Fig. 319). A median vagina is formed by a groove-like gutter in 

 the floor of the atrial cavity, which 

 becomes shut off in front but 

 remains open behind. 



Besides the mesoderm that 

 originates from the mother cells of 

 the mesoderm, mesenchyme cells, i.e. 

 wandering cells, are budded into 

 the blastocoele from the ectoderm at 

 two points on the posterior aspect of 

 the aboral surface of the larva. 

 These cells become transformed into 

 the larval musculature and corre- 

 spond to the mesectoderm of 

 Annelida, Mollusca, and Gephyrea. 



Lebedinsky 's statements have 

 been received with a good deal of 

 scepticism. It is possible that the 

 excretory organs do not arise as a 

 pair of ducts connecting the coelornic tes, testes. 

 cavities with the exterior, but are 



true nephridia ; on the other hand, Hatschek saw and figured the 

 mesodermal bands arising from the mesoderm mother cells, and it is in 

 accordance with all that we know of other groups that these bands 

 should give rise to the genital organs. 



We owe the most recent descriptions of the free-swimming larva 

 to Seeliger (1906) and Czwiklitzer (1909). Both of these authors 

 are anxious to demonstrate the exact correspondence between this 

 larva and Cyphonautes. Both point out that the cells forming the 

 floor of the atrial cavity are glandular, and compare these glandular 

 cells to the glandular cells of the " sucker " of Cyphonautes. Czwik- 

 litzer points out further the interesting fact that the dorsal organ is 

 only at the bottom of a ciliated pit in the retracted condition of the 

 larva. When the larva is paralysed by adding cocaine to the water 

 in which it swims and is then preserved, the brain is exposed on the 

 surface and the ciliated canal is flattened out. Both Seeliger and 

 Czwiklitzer compare the brain to the pyriform organ of Cyphonautcs. 

 The justice of this comparison will be considered later. Czwiklitzer 



VOL. i 2 u 



ex 



FIG. 319. Section of the 'embryo of 

 Pedicettina echinata, parallel to the 

 sagittal plane but lying to the right 

 of it. (After Lebediutsky.) 



nt, atrium ; ex, excretory organ ; ex.d, 

 excretory duct ; uv, ovary ; st, stomach ; 



