2952 THE WORM-LIKE PROTOCHORDATES 



sinks below the surface, only opening at the latter by small slit-like pores, and thus 

 their true characteristic is obscured in superficial view." On dissection, a rod-like 

 structure, which arises as an outgrowth of the alimentary canal above the mouth, is 

 seen projecting into the interior of the proboscis; and this rod has been identified 

 with the vertebrate notochord. Above this rod, and extending farther back, is a 

 dorsal nerve chord, corresponding to the vertebrate nerve tube, and having, as in 

 the latter, a central canal, at least during the earlier stages of growth. Some dis- 

 tance behind the notochord this nerve tube gives off a descending branch, connect- 

 ing it with a similar chord lying on the ventral aspect of the animal. 



We thus have evidence of the existence in this strange worm-like creature of 

 three essentially vertebrate characteristics, namely, gill slits, a notochord, and a 

 nerve tube; and it is not a little remarkable that while in the sea squirts the 

 notochord is found in the transitory tail, in Balanoglossus it is situated in the ante- 

 rior extremity, where it extends some distance in advance of the mouth. Quite 

 recently it has been shown that the torn aria larva of one species of Balanoglossus 

 also possesses an endostyle (see p. 2948), comparable to that of the lancelet and sea 

 squirts. That the creature under consideration is closely allied to the other Proto- 

 chordates, and thus to the Vertebrates, may be considered fairly certain; but there 

 are also indications of affinity with Nonchordates. In the first place, while certain 

 species of Balanoglossus pass through the earlier stages of their existence without 

 undergoing a metamorphosis, in other kinds such a transformation takes place; the 

 young making its first appearance in the world in the form of what is known as a 

 tornaria larva, or one closely resembling that of a starfish. And it is held by com- 

 petent naturalists that this resemblance must be indicative of some kind of genetic 

 relationship between Balanoglossus on the one hand, and starfish and sea urchins 

 on the other. In the second place, there are not wanting indications of affinity 

 with the so-called Nemertine worms, described in the next volume; these re- 

 semblances presenting themselves in the structure of the outer layer of the 

 skin, the presence of a proboscis (kept retracted in the Nemertines), as well as 

 in regard to the nervous system, the reproductive organs, and the alimentary 

 canal. 



The two other forms included among the Protochordates are respec- 

 her Forms t j ye j y j cnown as Cephalodiscus and Rhabdopleura, and bear the same 

 relation to the last as is presented by the compound ascidians to the lancelet. Both 

 these curious creatures are fixed forms, living in societies, reproducing their kind 

 by means of buds, and having a U-shaped, instead of a straight, intestine. Both 

 are likewise deep-water creatures, the former having been dredged in the straits of 

 Magellan at a depth of two hundred and forty-five fathoms, while the latter has 

 been taken off the Shetlands in ninety, and off the Lofoten islands in two hundred 

 fathoms. Extremely minute in size, Cephalodiscus lives in colonies, the individuals 

 wandering about the tubes of a common house, the walls of which are composed of 

 a gelatinous material, covered with spiny projections, and perforated by numerous 

 apertures for the free circulation of water. The mouth is overhung by a large 

 shield-like plate, surmounted by the row of plume-like tentacles; while on the side 

 of the body is a pedicle from which grow the buds; locomotion being probably ef- 



