ZOOLOGY AND BOTANY, MICKOSCOPY, ETC. 243 



suggests that the behaviour of the blastopore in PJioronis is obviously 

 due to the attainment of a terminal anus ; if we suppose the long axis 

 of the body to increase still more rapidly, while the posterior part of 

 the blastopore remains termina], and, at the same time, suppose that 

 the importance of a complete alimentary canal is not equal to the 

 importance of the body form, " then the tendency of the endoblast to 

 divide iuto anterior and posterior jwrtions attached to anterior and 

 posterior parts of the blastopore respectively might be consummated." 

 When the anterior endoblastic mass is the larger we get the so-called 

 oral blastopore, and when the posterior, the anal. If invagination 

 of different endoblastic masses is not synchronous the extremes will 

 give us stomodoea and proctodoea. 



Plioronis, then, is the first step towards a complete division of the 

 blastopore, and the inducing cause of this is the elongation of the 

 body, while the endoblast is still in an embryonic condition. 



2. The necessity of an early attainment of a terminal anus caused 

 the ectoderm to grow more rapidly than the endoblast, and resulted 

 in a division of the mesoderm into anterior and posterior parts ; this 

 division had as results the masking of the original mode of mesoderm 

 formation, and, 



3. Metameric segmentation. 



Structure of the Body-wall in Earthworms.* — F. E. Beddard 

 describes the structure of the body-wall of Perionyx and Perichceta. 



In Perionyx the epidermis closely resembles that of the common 

 earthworm. The latter, however, is only vascular in the region of 

 the clitellum, while in the former the terminal branches of the blood 

 system penetrate within the epidermis of the general body surface. 

 This intra-epidermic vascular plexus is entirely unrepresented in the 

 common earthworm as well as in the two genera Urochceta and 

 Pontodrilus. The epidermis of Perionyx agrees with that of the leech, 

 and differs from that of all other earthworms, in being pigmented. 



In Lumhricus the epidermis is separated from the subjacent layer 

 by a thin elastic membrane, from which are given off a number of fine 

 processes, which ramify between the individual fibres of the transverse 

 muscular coat. In both Perionyx and Perichceta this elastic network 

 is very highly developed: its function is probably to assist in ex- 

 tending the fibres after contraction. This structural peculiarity of 

 the muscular layers is not found in all Oligochseta. 



Excretory Organs of Worms.f — A. Lang seeks to prove that 

 the organs of secretion of the Platyhelminthes and particularly those 

 of the Tricladea constitute the point of departure for those of the 

 greater number of the higher worms. 



Amongst the Annelida and in Dinophilus the segmental open- 

 ings of the renal organ of the Tricladea are preserved, but the 

 longitudinal canals have disappeared. In Dinophilus, the internal 

 extremities of the segmental organs still present the vibratile cells 

 characteristic of the Platyhelminthes, whilst in the Annelida cells of 



* Proc.R. Phys. Soc. Edinburgh, viii. (1884) pp. 89-91. 

 t Arch. Sci. Phys. et Nat., xii. (1884) p. 432. 



R 2 



