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



the rudimentary,, ■notocliord 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 Phoroms, again, a somewhat similar condition to that in Cephalodiscus is present, 

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

 tentacles, and which, 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. Harmer. In the stalked or adult Loxosoma, 

 again, the organ is absent, a condition due, Mr. Harmer thinks, to the fact that the larval 

 form dies after giving off buds. A very different condition, however, occurs in Cephalo- 

 discus, in which the young buds soon pi-esent this and all the other organs of the adult, 

 although it is true the development 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 suppressed, and that we may have only a much modified 

 peripheral system remaining. 



Sense-Organs. 



In the preliminary account of Cep>halodiscns, 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 M 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 Rhabdopleura, 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 such is certainly peculiar 

 for organs of vision, but if Rhabdopleura 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 Loxosomse; indeed, they cmake 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, Harm«r describes a 

 pigment-spot on each side of the larval brain (even when the lumen is still '^reseat ^fter 

 involution). They consist of crescentric reddish-brown masses of pigment-, ^ with ^ 



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



