574 SEMIVERTEBRATES. 



surface, only opening at the latter by small slit-like pores, and thus their true 

 character 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 distance 

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

 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 

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

 Quite recently it has been shown that the tornaria-larva of one species of Balano- 

 glossus also possesses an entostyle (see p. 570), comparable to that of the lancelet 

 and sea-squirts. That the creature under consideration is closely allied to the 

 other Protochordates, 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 transforma- 

 tion 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 competent 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 resemblances 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 

 Otlier Forms. 



respectively known as Ceplialodiscus and Rliabdopleura, 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, repro- 

 ducing 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 Shetland's in ninety, and oft' 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 effected by means of this pedicle and the 



