608 INVERTEBEATA CHAP 



later stages of development into earlier stages of ontogeny. This 

 phenomenon, termed heterochrony by Lankester, which meets the 

 comparative embryologist at every step of the road on which he 

 travels, is one for which no adequate explanation has as yet been 

 suggested, but it seems to us that it is of the utmost interest in 

 weighing theories of heredity and of the origin of variation. 



In conclusion, therefore, we maintain that the asymmetries of the 

 larva of Amphioxus are due to the existence of a former " Hat-fish " 

 condition, but that these asymmetries have been reflected backwards 

 into a previous free-swimming condition so as to appear in the 

 pelagic larva. 



We have now placed lief ore the reader the arguments, which 

 seem to us convincing, that the Enteropneusta are related to the 

 vertebrate phylum. As we have already stated, however, all zoologists 

 are not in accord with this view. We have not here discussed the 

 objections which have been raised to it, because, in the second volume 

 of this work, the reader will have an opportunity of learning at 

 first hand the arguments on the other side, and will then be best 

 able to weigh both sides of the case for himself. 



UEOCHORDA 



The group of Urochorda, more familiarly known as Tunicata, 

 comprises animals, the vast majority of which are sessile, and many 

 of which have much the same external appearance as sponges. By 

 the older zoologists they were classed, with Polyzoa, as Molluscoida. 

 It never entered into the imagination of any one that they had 

 affinities with the Vertebrata till Kowalevsky (1867) showed that the 

 larva possessed a notochord, a tubular dorsal nerve cord expanding in 

 front into a brain-vesicle, and a pair of gill-slits. The larva has a long 

 post-anal tail to which the notochord is confined, and on this account 

 the name Urochorda (ovpd, a tail) is given to the group. Since a 

 brain-vesicle has been developed at the anterior end of the spinal 

 cord, and since there is a tail, we are driven to conclude that the 

 Urochorda must have branched off from the Vertebrate stem when 

 the Vertebrate stock had evolved far beyond the level represented 

 by the Cephalochorda ; for in this latter group there is a mere 

 rudiment of a brain-vesicle, and the tail is only beginning. 



The Urochorda include a few minute forms termed Larvacea, 

 which retain tail, notochord, neural tube, and brain-vesicle throughout 

 life, and remain free-swimming. About the development of these 

 animals little or nothing is known. There is also a small group 

 termed the Thaliacea which has secondarily acquired free-swimming 

 habits; the larval tail and notochord are lost in the adult, but the 

 sphincter muscles, which in ordinary forms surround mouth and 

 anus, are developed into hoop-like bands, and by the peristaltic action 

 of these muscles progression is effected. 



The vast majority of Urochorda, termed the Ascidiacea, are 



