398 SUMMARY OF CUKRENT RESEARCHES RELATING TO 



developmental steps in the evolution of the neuromuscular mechanism of 

 the higher animals. 



The nervous system is not limited to the ectoderm and to the 

 endoderm, but in certain regions it penetrates the supporting lamella 

 and thus connects one layer of the body with another. The supporting 

 lamella thus comes to contain nervous elements. 



Effector Systems of Actinians.*— G. H. Parker finds that llcfriditm 

 responds to environmental changes by at least four systems— the 

 mucous, the nematocyst, the ciliary, and the muscular. The first three 

 are independent effectors and are not under the control of a nervous 

 mechanism. 



The muscular system shows a variety of conditions. Some muscles, 

 such as the longitudinal muscles of the acontia, are independent 

 effectors and are not under nervous control. Others, hke the circular 

 muscles of the column wall, may act independently or under the 

 influence of nerves. Still others, such as the longitudinal muscles of 

 the mesenteries, act only in response to impulses from a relatively 

 complex nervous mechanism. 



Non-nervous muscular responses are carried out sluggishly and 

 require a minute or more for completion. Nervous muscular responses 

 are relatively rapid and may be accomplished in a second or so. 

 Notwithstanding that the whole musculature exhibits a high degree of 

 tonicity, there are responses, such as the expansion of the oesophagus 

 by the action of the transverse muscles of the complete mesenteries, 

 which are of the nature of well-individualized reflexes. 



Larval Actinian Parasitic in Rhizostome.t— C. Badham has found 

 the larvfe of Peachia hiUi parasitic in a Rhizostome up to the stage at 

 which they have been found free-Hving. The parasitic larva hitherto 

 described live on the exterior of their host, or in the gut, which opens 

 freely by a mouth. The larvae of P. hiUi, however, live for a con- 

 siderable period in the radial canals of Grambessa mosaica, a large 

 Medusa common on the coasts of New South Wales. The larvae 

 occurred in about every tenth Medusa examined during September and 

 October. In the latter month they were observed in the act of escaping 

 from their host through a hole, regular in outline, made in the sub- 

 umbrellar wall of the gut. Some were found with their oesophageal end 

 protruding from the hole, others which had already escaped were 

 adhering to the tentacles of their host. In the genus Peachia there is 

 a single deep siphonoglyph, the lips of which come together and form a 

 tube running from the enteron to the exterior, and in some species the 

 external opening is surrounded by a series of lobes forming a conchula. 

 Studying living larvse still attached to the gut-wall of the host, the 

 investigator found that when the larva is attached by the oesophageal 

 folds it is through the conchula that a constant stream of fluid-bearing 

 food -particles goes to the enteron. He suggests that the larvEC of the 



* Journ. Exper. Zool., xxi. (1916) pp. 461-84. 



t Quart. Journ. Micr. Sci., ii. (1917) pp. 221-9 (3 figs.). 



