NOTES ON THE STRUCTURE OF OIKOPLEURA. 33 



dots being evidently the deeply stained nuclei of the small 

 spermatic cells (see PL I. figs. 3 and 5). 



After about 20 sections from the posterior end the 

 lateral lobes of testis are found to be encroaching upon 

 the ovary in its centre so as to divide off a smaller dorsal 

 lobe from the ventrally placed main mass (PL I. fig. 2). 

 In this region is found the apparently single genital duct. 

 The dorsal ectoderm bends down in section 24 to form 

 a slender epithelial tube v^hich seems to join the v^all 

 of the ovary (PL IV. fig. 9, o.d.). The testis lobes come 

 in contact dorsally at this point and may possibly join 

 (PL I. fig. 3), so the tube might be vas deferens, or act both 

 as oviduct and vas deferens. The dorsal lobe then dies 

 out after a fev7 sections, and the ovary is then represented 

 by a nearly flat ventral plate v^hich can be traced forwards 

 to about the 90th section (counting from the posterior end). 

 Figure 3 on Plate I. shows the typical arrangement in a 

 number of sections, say numbers 25 to 45, where the 

 ventral ovary and the two lateral testis lobes enclose a 

 small triangular area, which will be occupied a little 

 further forward by the alimentary canal. The whole is 

 still surrounded by a delicate ectoderm with no trace of 

 a distinct test. 



The next figure (PL I. fig 6) shows, in the 48th section, 

 the most posteriorly placed part of the alimentary canal. 

 It is a short dorsal diverticulum from the point where the 

 oesophagus joins the stomach, and can be traced forwards 

 through the sections into the oesophagus and then the 

 pharynx or branchial sac. In about the 50th section we 

 come upon the first trace of the test in the form of a thin 

 layer of cuticle dorsally placed and co-terminous with an 

 area of rather larger ectoderm cells. As one traces the 

 sections forwards these ectoderm cells get larger, and the 

 layer of test over them becomes thicker (see PL I. figs. 7 



