ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 263 



plates on the disk, change with age. It is to be suspected that many 

 so-called species of Ophiuroids are only growth-stages. 



Evisceration and Regeneration in Thyone briareus.* — John W. 

 Scott has made a very interesting study of this Holothurian. The 

 evisceration affects oesophagus, stomach, intestine, calcareous ring, nerve 

 ring, tentacles, ring canal, Polian vesicles, stone canal and madreporite, 

 and the retractor muscles of the oesophagus. The method used to pro- 

 duce evisceration was to allow the animal to stand in stagnant water for 

 some time, and then to flood it with running water containing much 

 oxygen. Alternating these processes produced as many as 65 p.c. of 

 self-mutilated individuals. It has been suggested that breakages occur 

 where there are accidental structural weaknesses, but at times the skin 

 of Thyone appears to dissolve away with little or no pressure present, 

 and retractors frequently break off by local constrictions instead of by 

 longitudinal pull. 



The eviscerated parts of Thyone showed for a time great irritability 

 and could be kept alive for some time. The part remaining was less 

 responsive, but reacted to tactile stimulus, and to lack of oxygen. As 

 regards regeneration, it was found that all lost organs may be regener- 

 ated. But this occurred only when all parts concerned in evisceration 

 were completely expelled. Otherwise, the animal dies. During the 

 process of regeneration, the behaviour gradually becomes more responsive 

 and is finally like that of the normal individual. This appears to be 

 connected with the growth of a new nervous system. 



It appears that while Thyone is functionally a bilateral animal, the 

 most conspicuous individual differences involve structures that have a 

 radial arrangement. The Polian vesicles vary greatly in number, in 

 size, and in location. There is a strong tendency for these to occur 

 on the left side, and this arrangement is undoubtedly due to ancestral 

 conditions, for the present bilateral habits of Thyone are not likely to 

 have had any influence in producing the asymmetry in question. The 

 retractor muscles in a single radius consist of single or multiple strands, 

 and this variation is closely correlated with a similar variation in the 

 number of Polian vesicles. It was found from the study of a number of 

 specimens that individual peculiarities of structure tend to be reproduced 

 in regeneration, even against the generalized ancestral influence. 



It is evident that autotomy enables Thyone to survive for a con- 

 siderable period on a smaller than normal supply of oxygen. Never- 

 theless, the conditions which seem to give rise to self-mutilation have 

 all a pathological aspect. 



The conditions in Thyone afford some evidence for believing that 

 when this animal abandoned the fixed stage the Polian vesicles con- 

 formed more or less to the radial type. The author maintains that 

 the present arrangement of Polian vesicles in Thyone can be best ac- 

 counted for on the theory of phylogenetic influence. Those vesicles 

 have retained their most complete radial arrangement in those species 

 of Echinoderms which have maintained to a high degree the functional 

 activity of the water- vascular system. 



* American Naturalist, xlviii. (1914) pp. 280-307. 



