264 LECTURE Xll. 



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

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

 Terebella 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 antennae. 



Thus, in the class Annulafa, 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 

 parent: it indicates, only in a very transient w^ay, 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 embryos in some viviparous Eri*antia, e. g. 

 Eunice and Cystonereis, ai'e 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 propagated by gemmation and 

 spontaneous fission. Should these observations receive confirmation 

 and extension, they will show that such Annulata 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 Acalephce. 



Class ANNULATA. 



Body soft, symmetrical, vermiform, annulatcd ; with suckers, sela) 

 or setigerous tube-feet. Alim ciitary caral wilh two apertures and 

 proper parietes. A vascular sys tcm circulating, in most, red blood: 

 respiratory organs. Ganglionic double nervous chord. 



