306 A. T. MASTERMAN. 



mention. Jnst before the anus is reached the lumen of the 

 gut widens out into a larger vestibule, which may be termed 

 the rectum. 



The alimentary canal thus consists of a wide mouth leading 

 into an " oesophagus/^ which is most likely epiblastic, and 

 should be more correctly termed the stomodseum ; the pharynx, 

 with its notochords, leading by a short oesophagus to the 

 stomach, with its two " digestive areas;" and the intestine, 

 with the distal end widened into a rectum and terminating in 

 a posterior anus. 



Organs of the Mesoblast. 



The mesoblastic structures of Actinotrocha are highly 

 developed, and indicate that this larva must be regarded in 

 its highest development as at a high morphological grade. 

 Thus there is little indication of '^ mesenchyme,'' and the 

 coelom is present with splanchnic and somatic layers. In 

 addition to this the vascular and muscular svstems are well 

 differentiated. 



Coelom. 



The mesoblast is in the condition of a thin layer of proto- 

 plasm (PL 19, figs. G — 10, and 15, &c.), with a few nuclei 

 dotted here and there, but at certain places this layer is thick- 

 ened by a massing together of mesoblast cells which form 

 simple contractile muscle-cells, and at others again are found 

 protoplasmic processes of the same nature, extending across 

 the body cavity. This thin layer of mesoblast is closely 

 applied to the hypoblast along the gut, except at certain places 

 where the blood lacunse are formed by the spaces left between 

 the two layers. On the outer wall the somatic mesoblast is 

 also in close contact with the epiblast, though between them is 

 a thin layer of mesoblastic chondroid tissue. 



The coelom is segmented into three parts, — the coelomic cavity 

 of the pre-oral lobe, that of the collar, and that of the trunk. 



The cavity of the pre-oral lobe (PI. 19, fig. 6) is of the same 

 size and shape as the hood itself. Posteriorly it is produced 



