PTEROBRANCHIA 2 1 



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, 1 or from scattered 

 mesoderm cells 2 which lie in the segmentation-cavity. 



The metamorphosis of Tornaria is accompanied by a great 

 diminution in size, 3 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, 4 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 

 {^-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 is 

 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 T. agassizii}. 

 - Morgan (in Balanoglossus biminiensis}. 



3 A similar shrinkage takes place in the metamorphosis of the larva (Lepto- 

 cephalus} of Eels, as has been shown by Grassi, Quart. J. Micr. f!ci. 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. 



