142 HOMOLOGIES. 



long enough retained in the ovisacs to develop their contents, after impregnation through the 

 lateral (sexual) pores. 



In the homologies of no organ, however, does the separation between the Planarians and 

 Nemerteans become more apparent than in those of the proboscis, which, with all its adjuncts, 

 appears to be a structure purely Nemertean. 1 Its definite aperture in front, its relation to the 

 ganglionic commissures (between which it passes), its remarkable microscopic structure, and 

 distinct muscular sac or sheath containing the highly organized corpuscular fluid, all point it out 

 as an organ mi generis, and apparently without prototype or homologue in the Planarians or their 

 allies. As already mentioned, I am inclined to consider the proboscis of the Planarian as the 

 analogue and homologue of the oesophageal division of the digestive tract in Linens and 

 Anpkiporus, and the " Schlund" of Vortew. The diminished size and atrophied condition of the 

 proboscis in Brorhynchus seem to lead on the Nemertean type to certain of the Rhabdoccela. 



Both groups are characterized by great recuperative powers after injury, new parts and 

 organs replacing those that have been cut off; while mere fragments not infrequently grow into 

 perfect animals. 



Both consist for the most part of predatory and carnivorous creatures, that, notwithstanding 

 their general deprivation of organs of offence and defence, manage to prey on animals much 

 higher in the scale of organization than themselves, such as the Annelida. Their habits are 

 also in many respects similar. 



Bipalium. 



I thought that considerable light might be thrown upon the affinities of the Planarians and 

 Nemerteans by an examination of Bipalium, whose elongated body and central mouth indicated 

 the probability of its intermediate position. 



In Bipalium 2 there is externally (in the preserved condition) a rather dense cellular cutis, 

 similar in structure to the same coat in the Planarians and Nemerteans, though less defined from 

 the subjacent investment, which consists in this case of a thin belt of circular muscular fibres. 

 There next occurs a longitudinal muscular layer, split into isolated fasciculi, between which 

 certain pigmentary and cutaneous elements and connecting fibres lie. Thus the coat in 

 transverse section presents a barred appearance, especially in the dorsal region, where the pigment 

 is most marked, the dark band being interrupted by the pale longitudinal fasciculi. In super- 

 ficial longitudinal sections, also, the same aspect is caused as in Linens by the intrusion of the 

 pigmentary and cutaneous elements amongst the muscular. The intermediate region below 

 the coat just mentioned has numerous cells and granules amongst the fibres which connect it 

 with the next layer and the general stroma of the body-cavity. There are also many cells, often 

 of a flask-shape, with the narrow end external, filled with long and somewhat spindle-shaped 



1 It is probable some further light will be thrown on the homologies of this organ in the anatomy 

 of the Annelida. 



3 I am indebted to Prof. E. P. Wright, of Dublin, for the opportunity of examining this form, 

 which was kindly placed in my hands along with many foreign Annelida collected by himself. His 

 genus Dunlopea is synonymous with Bipalium. 



