3o8 NATURAL SCIENCE. Nov., 



Laodice and its allies, true endodermal sense-clubs are present, 

 differing only from those of the Trachylinae in the absence of 

 concretions. 



The Senses of Medusae. 



In the same paper Mr. Brooks discusses the physiological nature 

 of the sense-organs of Laodice and other Medusae. He thinks it 

 probable that the sense-vesicles of Medusae, Mollusca, Crustacea, 

 Brachiopoda, Doliolum, and many other invertebrates, may give 

 sensations of sound, but that it by no means follows that this is their 

 only or primary function. In the case of eyes, he thinks it easy to 

 believe that all incipient stages of evolution have been related to light. 

 Indeed, we know that protoplasm itself responds to light, and the 

 various stages of mechanical combinations of sensitive cells, pigment 

 and transparent cells, and focussing cells, are not difficult to imagine. 

 But hearing is different ; and he suggests that change of function has 

 played a part in the evolution of auditory organs, since sound has 

 no direct effect upon protoplasm. 



Mr. Brooks suggests that possibly the primitive object of these 

 sense-organs with their concretionary masses was to give a sense of 

 gravity. Medusae are animals nearly of the same specific gravity as 

 the fluid in which they live. An obvious way in which they could 

 appreciate special relations would be if delicate weights were placed 

 round their margins in close relation to the nerve-ring. The slightest 

 tilting would cause a different impression upon the nerve-ring at the 

 different parts of the circumference, as the mobile organs containing 

 the heavier concretions would press differently upon the circum- 

 ference. "Among the veiled Medusae the sense-clubs are found in 

 three different stages of perfection : 



" I. In the Thaumantidae and Cannotidae they are simple clubs, 

 with an enlarged tip, united by a narrow stalk to a sensory eminence 

 on the nerve-ring. 



" 2. In the Narcomedusas and in the Aglauridae among the 

 Trachymedusae, the enlarged club-shaped tip of the projecting club is 

 loaded with calcareous concretions. 



"3. In most of the Trachynemidae the sensory eminence is raised 

 up around the club in such a way as to enclose it in a sensory vesicle, 

 which, in the Geryonidae, is embedded in the gelatine of the bell." 



Comparison of the figures given by Mr. Brooks with those which 

 the Hertwigs give (in their memoir of the nervous system and sense- 

 organs of the Medusae) will show that we here have the same organ 

 in three successive stages of perfection. It is clear that each suc- 

 cessive stage is adapted for increasing its efficiency as a weight organ, 

 although it is only in the last and most perfected stage that it affords 

 any basis for the analogy with the " auditory organs " of other animals 

 upon which the Hertwigs base their belief that the sense-clubs are 

 ears. 



