TROCHOPHORE OF HYDHOIDES UXCINATUS (eUPOMATQS). 579 



gut are somewhat longer and more powerful than those of the 

 stomach. Immediately above the proctodteum the lumen of 

 the gut is narrowed down bj a projecting ridge. This is 

 well shown in the trochophore ot' Hydroides pectinata 

 (fig. 18). Below this constriction the gut opens into the 

 proctodeum, which, like the stomodreiim, also secretes a fine 

 cuticle. At first the cells of the archenteron are uniformly 

 cubical in appearance, but those of the oesophagus and the gut 

 soon thin out, while those of the stomach alone retain their 

 primitive appearance. 



The larva at this time has the shape represented in figs. 

 9, 10, and 12. The anal vesicle begins to appear as a small 

 vacuole in one of the ectoderm cells of the anal region. This 

 at first connects with the exterior by a small duct, but this 

 soon closes, and the vesicle increases rapidly in size. The 

 cytoplasm oi the cell sti-etches so that a thin envelope alone 

 is left which surrounds the vesicle. It then becomes con- 

 stricted into two portions, as shown in fig. 14. The original 

 nucleus of the cell is seen projecting into the bhistocoel from 

 the upper wall of the vesicle. 



On either side of the gut, just above the anal vesicle, a 

 large conspicuous nucleus is seen embedded in the wall. 

 This is the nucleus of the coelomesoblast cell. In the stages 

 represented in figs. 9 and 10 it is not so prominent as in the 

 later stages shown in tigs. 12, lo, 15, and 16. As develop- 

 ment proceeds it is pushed out more and more into the 

 blastocoel. In Text-fig. 19 it appears to be free in the 

 blastocoel, but examination of the subsequent sections of 

 this series clearly shows it to lie in the gut-wall. As I have 

 mentioned, it is of somewhat different staining reaction to 

 the surrounding cells, and this contrast is shown somewhat 

 in this text-figure, which is from a camera drawing of an 

 actual section. The section passes a little to one side of the 

 median line, and is slightly oblique, as the mouth and 

 oesophagus are cut in the median plane, while the section 

 passes through the lateral wall of the stomach and the gut. 

 In Text-fio\ 20 is shown a section of an older stage in which 



