144 PHYSIOLOGY. 



The two forms of sense-l)0(lies now described are never associated in the same species, and 

 the case of Tyaropais, as already shown, affords no exception. It has been already stated, as a 

 nearly universal rule, that the ocelli are confined to the true sexual medusa or gonocheme while 

 the lithocysts are found only in the blastochenie. Exceptions to this rule, however, are afforded by 

 Tliauinanlim, as limited by Gegenbaur, which, though a blastoclieme, has ocelli instead of litho- 

 cysts, and by Staurophora and LaoiUcea, which are also blastochemes, though, according to 

 Agassiz, the ocellus takes the place in them of the lithocyst. On the other hand, Goodsirea, 

 Wright, has, according to Wright's observation,^ lithocysts instead of ocelli, though it is a true 

 gonocheme. MeUcertiim, another blastochenie, is, according to Agassiz, also very exceptional, 

 for neither lithocysts nor ocelli are found in it. 



The ocellus, however, is by no means so constantly present in the gonocheme as the lithocyst 

 is in the blastocheme, many planoblasts, with the other characters of the gonocheme, having no 

 definite pigment spot, though the bulbous expansion at the base of the tentacles, with its 

 contained coloured matter, has in such cases been frequently confounded with a true ocellus. 



Touch. — Analogy would lead us to regard it as probable that sensitiveness to touch resides 

 in all parts of the surface of the Hydroida — such parts, at least, as are not under cover of a 

 thick perisarc. It would seem, however, that this sensitiveness is in a special degree conferred 

 upon the tentacles of both hydranth and medusa. The slightest contact of the tentacles of 

 the hydranth with a foreign body not destined to be appropriated as food, will frequently be 

 followed by the instant contraction of the entire hydranth, accompanied, in the calyptoblastic 

 hydroids, by retraction within the hydrotheca, an action which is plainly subservient to the 

 safety of the animal by causing its withdrawal from sources of injury. The ordinary contact, 

 however, of pabulum with the tentacle, excites only the prehensile functions of one or more 

 tentacles, and is not accompanied by any general contraction of the hydranth. In like manner 

 the marginal tentacles of the medusae instantly contract on being touched. We are not, however, 

 obliged to conclude that these acts are necessarily accompanied by consciousness on the part of 

 the hydroid. Like many other apparently conscious acts in the animal economy, they may be 

 purely reflex and automatic. 



Corda and Wright have described as organs of touch the minute bristle-like bodies to which 

 the latter has given the name of palpocils (see above, p. 107). Haeckel" has also described as organs of 

 touch certain long fine and stiff hairs (" Tastborsten"), which he has observed in Cunina, and which 

 Gegenbaur^ and Keferstein and Ehlers* have observed in OSginela ,- they are developed from the 

 epithelium lying over the ganglion-like swelling on which the lithocyst is seated in these medusae ; 



Loud.,' vol. ill, p. 22), who adduces in favour of this view the action of the lithocyst on polarized light, 

 when the concretion exhibits a well-defined black cross, indicating, as lie believes, a gradually decreas- 

 ing density from the centre to the circumference, and pointing to a close resemblance between these 

 bodies and the crystalline lens of higher animals. Evidence of the sensibility of the hydroid plano- 

 blasts to light is found in their habit of congregating on the light side of the jar in which they may be 

 confined. I have noticed, on the other hand, that the planulse of Plunmtaria pinnata, in which, how- 

 ever, nothing like sense organs exist, avoid the light, always crowding towards the dark side of the jar. 



^ 'Edinh. New Phil. Journ./ July, 1859. 



" ' Die Familie der Riisselquallen,' p. 137. 



' ' Versuch eines Systenies der Medusen,' p. 266. 



* ' Zoologische Beitrage,' p. 94. 



