1 PTEROBRANCHIA 2p At 
The body-cavities are formed as five derivatives of the 
archenteron. One of these is unpaired, and becomes the 
proboscis-cavity ; while the others are the paired cavities of 
the collar and trunk (cf. Fig. 2). There is some uncertainty 
about the origin of the body-cavities of the free-swimming 
Tornaria, although it seems most probable that they are developed 
either from the wall of the stomach or intestine, or from scattered 
mesoderm cells ® which lie in the seementation-cavity. 
The metamorphosis of Tornaria is accompanied by a great 
diminution in size,° probably due to the loss of water; by this 
cause and by the simultaneous thickening of the skin, the larva 
loses its transparency. 
The external features of the metamorphosis are sufficiently 
indicated by Fig. 8, the ciliated bands finally disappearing. 
The dorsal pore persists as the proboscis-pore; the notochord 
and numerous gill-slits are developed as outgrowths of the 
alimentary canal, the reproductive organs make their appearance, 
probably from the mesoderm,* the trunk meanwhile elongating 
so that the proportions of the adult are acquired. 
Order II. Pterobranchia. 
Tubicolous Hemichordata, with one pair of gill-slits or none, a 
U-shaped alimentary canal, and a dorsal anus situated near 
the mouth. Proboscis flattened ventrally into a large “ buccal 
disc,’ its base covered dorsally by the collar, which %s 
produced into two or more tentaculiferous arms. Trunk 
short, prolonged into a stalk. Reproduction by budding 
Occurs. 
This group consists of the two genera Cephalodiscus (Fig. 9) 
and Rhabdopleura (Fig 12). The latter, first dredged by G.O. 
Sars, in 1866, from 120 fathoms off the Lofoten Islands, was 
included in a catalogue of deep-sea animals published by his 
father, M. Sars, in 1868 as Halilophus mirabilis, a name which 
1 Agassiz, Bourne, Spengel, Morgan (in 7. agassizit). 
2 Morgan (in Balanoglossus biminiensis). 
8 A similar shrinkage takes place in the metamorphosis of the larva (Lepto- 
cephalus) of Eels, as has been shown by Grassi, Quart. J. Mier. Sci. xxxix. 1897, 
p-. 374. 
4 Schimkewitsch, Zool. Anz. xi. 1888, p. 283; Morgan, J. Morphol. ix. 1894, 
p. 60; Punnett (op. cit. p. 661) believes that they are ectodermal. 
