626 PLANARIAN WORMS. ANNELIDA. 



Ova of the other, seems to be generally necessary. The Ovaria, as in 

 the Entozoa, extend through a large part of the body, their ramifica- 

 tions proceeding from the two Oviducts (k, Jc), which have a dilata- 

 tion (I) at their point of junction. — There is much obscurity about 

 the history of the Embryonic Development of these animals ; and 

 the facts observed by Siebold seem to be best explained upon the 

 hypothesis, that what has been usually considered as an Egg is 

 really an Egg-Capsule containing several Embryoes with a store of 

 supplemental Yolk, as in Purpura (§ 479), which Yolk is swallowed 

 by the Embryoes at a very early period of their development 

 within the capsule.* After their emersion from the Capsule, the 

 Embryoes bear so strong a resemblance to certain Infusoria, as to 

 have led Prof. Agassiz to the conclusion that the genera Para- 

 mecium and Kolpada are nothing else than Planarian larvae, — an 

 idea decisively negatived by recent discoveries (§ 347). The Pla- 

 narise, however, do not multiply by eggs alone ; for they occa- 

 sionally undergo spontaneous Fission in a transverse direction, 

 each segment becoming a perfect animal ; and an artificial division 

 into two or even more parts may be practised with a like result. 

 In fact, the power of the Planarise to reproduce portions which 

 have been removed, seems but little inferior to that of the Hydra 

 (§ 411) ; a circumstance which is peculiarly remarkable, when the 

 much higher character of their organization is borne in mind. 

 They possess a distinct pair of Nervous Ganglia (/,/), from which 

 branches proceed to various parts of the body ; and in the neigh- 

 bourhood of these are usually to be observed a number (varying 

 from 2 to 40) of Ocelli or rudimentary Eyes, each having its re- 

 fracting body or crystalline lens, its pigment layer, its nerve-bulb, 

 and its cornea-like bulging of the skin. The integument of many 

 of these animals is furnished with 'Thread-cells' or ' Filiferous 

 capsules,' very much resembling those of Zoophytes (§ 428). 



488. Annelida. — This Class includes all the higher kinds of 

 Worm-like animals, the greater part of which are Marine, though 

 there are several species which inhabit Fresh water, and some 

 which live on Land. The body in this class is usually very long, 

 and nearly always presents a well-marked segmental division, the 

 segments being for the most part similar and equal to each other, 

 except at the two extremities ; but in the lower forms, such as 

 the Leech and its allies, the segmental division is very indistinctly 

 seen, on account of the general softness of the integument. A 

 large proportion of the Marine Annelids have special Respiratory 

 appendages, into which the fluids of the body are sent for aera- 

 tion ; and these are situated upon the Head (Fig. 327), in those 

 species which (like the Serpula, Terebella, Sabellaria, &c.) have 

 their bodies enclosed by Tubes either formed of a Shelly substance 

 produced from their own surface, or built-up by the agglutination 



* See § 129 of Siebold and Stannius's " Vergleichende Anatomie ; " also 

 "Muller's Archiv.," 1850, p. 485. 



