26A LECTURE XII. 



developed in the same order, and a free-moving or " errant " anellld 

 ensues. Finally, the cilia of the buccal rings are lost, the young 

 TereLella reposes, and envelopes itself in a mucous tube. It is now 

 half an inch in length, and the circulation may be seen in the dorsal 

 vessel, the branchial filaments, and the cephalic antennjE. 



Thus, in the class Amiulata, the observations on development, so 

 far as they have extended, show that its progress varies in different 

 orders, and in a minor degree in different families. The embryo of the 

 leech, as soon as it is individualised, or made distinct from the germ- 

 mass, presents the characteristic forms and locomotive organs of the 

 jDarent: it indicates, only in a very transient way, during the develop- 

 ment of this form, some characteristics of the Turbellaria. In the 

 earth-worm the ultimate form is obtained by a successive develop- 

 ment of segments, without any previous free locomotion of the embryo 

 as a ciliated monad. The embxyos in some viviparous Errantia, e.g. 

 Eunice and Cystonereis, are at first acephalous, apodal, and abranchial, 

 but do not manifest a rotifer-like stage. The young of other dorsi- 

 branchiate and of some cephalobranchiate anellids, on the contrary, 

 start into free locomotive life under a form, and with instruments 

 and modes of locomotion, quite distinct from those that characterise 

 the adult. We may hence deduce the necessity for caution in 

 generalising as to the metamorphosis or non-metamorphosis of a par- 

 ticular class. A very interesting phenomenon in the development of 

 some of the Errantia is that which has been noticed in the young 

 of certain species when it has acquired the form of the parent : its 

 manifestation, viz., of a power of parthenogenetic propagation by 

 virtue of the retention of unchanged germ-cells or nuclei in the 

 penultimate and last segments of the body : it would seem, moreover, 

 that it did not develope male or female organs in its own person, but 

 that these are formed in the offspring pro{)agated by gemmation and 

 spontaneous fission. Should these observations receive confirmation 

 and extension, they will show that such An7iulata are subject to 

 what has been called "alternate generation," although somewhat 

 modified as compared with th at which has been noticed in the 

 Entozoa and jicalej)/ice. 



Class ANNULATA. 



Bodi/ soft, symmetrical, vermiform, annulated ; with suckers, seta; 

 or sctigerous tube-feet. Alim cntary ciiral with two apertures and 

 proper parietes. A vascular sys tcni ciit ulating, in most, red bleed : 

 respiratory organs. Ganglionic double nervous chord. 



