574 
SEMIVER TERRA TES. 
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; anti 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 
Other Forms. . . ° 
respectively known as Cephalochscus 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, 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 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 effected by means of this pedicle and the 
