^^^ ZOOLOGY. 



A NEW PEOnTIVE TYPE OF WORMS.* 



Tl7e genus Polygordms lias been supposed to represent one of the most 

 primitive forms of Annelids, but in 1880 Dr. Hatschek made known a 

 species which seems to be still more simple in its organization, and there- 

 fore to exhibit the most archaic form in which the Annelidan structure 

 IS known to exist at the present day. This new form has been named 

 Protodrilus LeucJcarti, and was found near Messina. The inferiority of 

 its organization is manifest especially in the nervous svstem, the ven- 

 tral ciliary groove, the blood vascular system, and the intestine The 

 muscular system resembles that of PohjcioriUus. " The nervous system is 

 difficult to make out in the living object, the ganglionic nature of the 

 fi-ontal ganghon being indistinguishable and the presence of the organ 

 merely indicated by a thickening of the integument ; the sensory organs 

 are represented by two transverse, elongated, ciliated slits, placed on the 

 dorsal surface of the anterior portion of the head. There are no pig- 

 mented eyes." The intestine has no rectal division. " Just behind the 

 mouth there opens into the oesophagus a muscular organ of a compli- 

 cated form, and terminating blindly in a chitinous vesicle very similar 

 to the same organ in Pohjgordius; it has a function which still remains to 

 be discovered." (Condensed from J. R. M. S., vol. iii, pp. 791-792.) 



POLYGORDIUS AND ITS RELATIONS TO OTHER WORMS. 



A worm-bke form that has excited much interest and has been 

 regarded as the most primitive of Annelids is the genus Polygordius. 

 What IS now known to be the larva of this form has been long known, 

 and had excited considerable speculation, but its parentage was discov- 

 ered but lately. Dr. Hatschek has been able to verify the develop- 

 mental stages of this type,t and has recognized six: (1) That prior to 

 segmentation, when the embryo is small and transparent, broader than 

 long and divided into two parts by two parallel circlets of cilia in the 

 niiddle line, with the mouth placed between these and the arms, and 

 the latter opening at the inferior pole of the body, and with twi eye 

 spots above; (2) the primitive segments of the body appear, the meso- 

 derm IS further developed, as is also the posterior circlet of cilia; (3) 

 the larva becomes elongated, differentiation of the ventral cord and 

 antennae ensues, and a ciliated pit supervenes on either side of the head 

 and represents apparently the future olfactory organ ; (4) the walls of 

 f^^^^^l^'VT^ ^'T' ^^^^^Stliened by the apposition of the proper 

 ^reS Iv^^l 1 mesoderm, and the posterior circlet of cilia becomes 

 ^-^^Z:^!I!^^I!!!LL(^)i^!^^ and the 



jB.10. z,ool. Inst. Wien, voL m, pp. 79-92, with ,2 pi 



vol. vi, pp. 324-326.) (Translated in Ann. Mag. Nat. Hist., (5,) 



