24 THE VOYAGE OF H.M.S. CHALLENGER. 



the rudimentary notochord will be specially referred to by Mr. Harmer in a note appended 

 to this paper. The view of Professor Sars that this structure cannot be a nervous ganglion, 

 because it does not lie in the substance of the body, would not seem to merit the 

 importance he attaches to it, when the condition of the great nerve-cords and ganglia of 

 Annelids, for instance, are considered. These are purely hypodermic in position, lying 

 between the latter and the basement-layer beneath. 



In Phoronis, again, a somewhat similar condition to that in Cephalodi!^cus is present, 

 the nervous concentration taking the form of a ring round the mouth at the bases of the 

 tentacles, and this, like the cord running along the foot, is epidermic (hypodermic) in 

 position.^ The same position (hypodermic) of the nervous system is found in Balano- 

 glossus, so that the relations of the nerve-centre are by no means exceptional. 



The position of this nervous centre would not appear to correspond with the larval 

 brain of Loxosoma as described by Mr. S. F. Harmer. In the stalked or adult Loxosoma, 

 again, the ganglion is subcesophageal, and is therefore not the homologue of the larval 

 brain. A very different condition, however, occurs in Cephalodiscus, in which the 

 young buds soon present this and all the other organs of the adult, although it is true 

 the develojjment and perhaps metamorphoses of the species from the egg are unknown. 

 It has also to be borne in mind that certain parts of the central nervous system may 

 have been suj)pressed, and that we may have only a much modified peripheral system 

 remaining. 



Sense-Organs. 



In the preliminary account of Cephalodiscus, the close relation of the so-called " eye- 

 spots" to the ovaries was duly pointed out, and recently the examination of more 

 satisfactory sections made with a microtome demonstrated at once their true nature'; viz., 

 that they are oviducts with thick pigmented walls. The resemblance of these structures, 

 both externally and in section, to a modified organ of sight, is one of the most remarkable 

 features in the animal. Their description will be given subsequently. 



In Rhahdopleura, Lankester mentions the occurrence of five spherical pigment- 

 corpuscles at the superior dorsal margin of the buccal disk, and regards them as rudi- 

 mentary sense-organs for the perception of light. The position of these is certainly peculiar 

 for organs of vision, but if Rhabdop)leura has a trace of the central nervous organ 

 observed in Cephalodiscus, these would readily be within reach of its communications. 



It is interesting that eyes occur in most larval forms of Loxosoma; indeed, they make 

 their appearance when the larva is still in the egg. They are situated under the hypoderm, 

 and resemble pigment-masses. In Loxosoma leptoclini, for instance, Harmer describes a 

 pigment-spot on each side of the larval brain (even when the lumen is stifl present after 

 involution). They consist of creseentric reddish-brown masses of pigment, with a 



1 Caldwell, Proc. Roy. Soc, vol. xxxiv. p. 372. 



