220 SUMMARY OF CURRENT RESEARCHES RELATING TO 



with sparse orange or brick-red pigment, are all discussed at length. 

 Tbo yellowish or greenish moniliform digestive tube, its openings, and 

 its ciliated interior; the body-cavity between the musculocutaneous 

 wall and the gut; the vertical septa dividing the cavity and other 

 oblique septa ; the contained colourless fluid with pigment corpuscles 

 and reproductive elements, then receive full attention. The lateral walls 

 of each segment include a pair of ciliated horizontal canals, com- 

 municating with the body-cavity by a funnel on the anterior face of each 

 septum. The vascular system consists of a dorsal, and of a ventral 

 vessel, usually connected at each septum by a cross branch. The ventral 

 vessel bifurcates in the cephalic segment, and unites with the dorsal. 

 Caudally the two vessels end in culs-de-sac. In P. neapolitanus 

 the lateral branches have vascular appendages. Tho blood is red in 

 P. lactens, green in P. crythrophthalmus, yellow in P. neapolitanus, 

 uncoloured in one of Rajewski's species. The cephalic lobe of the first 

 segment contains a central brain, which seems simple dorsally, but 

 laterally is bilobed, and ventrally trilobed. The ventral median line 

 bears a clear nerve-strand. The eyes are inconstant in the adults, and 

 at best, rudimentary. Possible auditory organs, present in larvse, do not 

 persist. The oblique septa of each segment bear paired sexual organs. 

 At maturity the contents fill the body-cavity. The sexes are separate. 

 All these facts are described at length. 



II. Development. — In the case of the female, at least (in P. neapoli- 

 tanus, and P. appendiculatus, not in P. villoti), sexual maturity appears 

 to be an end of the individual life ; the ova are liberated by 

 dehiscence. The appearance of the ripe ova is described. Artificial 

 fertilization was effected, but the intimate processes were not observed. 

 The segmentation is total, but unequal. From the stage with four 

 cells, macro- and micromeres can be distinguished. It seems that the 

 two primitive layers, epiblast and hypoblast, do not result respectively 

 from the two first blastomeres. One of them, probably the epiblast, is 

 formed at the expense of one of the two first spheres of segmentation, 

 plus a certain number of elements successively arising from the other. 

 The micromeres result in epiblast, the macromeres in hypoblast. The 

 mesoblast arises from the hypoblast. The blastula phase is apparently 

 succeeded by epibolic gastrulation, and the gastrula developes into a 

 trochosphere larva. Some of the last conclusions are more or less 

 hypothetical, but are confirmed by what is known of the development of 

 Protodrilus. 



After describing the external features of the larva of P. neapolitanus 

 at successive stages, Fraipont proceeds to a detailed analysis of the 

 various phases of larval metamorphosis. Starting from the trocho- 

 sphere, he describes six distinct stages, and collates them with those 

 described by previous investigators, and especially by Hatschek. The 

 organogeny is resumed separately, that of the nervous system starting 

 from (1) a central organ, the syncipital plate and the two lateral trunks 

 from it ; (2) the peripheral system, consisting of numerous nerves 

 whose multiple terminations are associated with superficial epidermic 

 cells ; that of the alimentary system from stomoda3um, mesenteron, and 

 proctodeum ; that of the body-cavity from the blastoccele ; that of the 

 mesoblast from two primordial mesoblast-cells arising from the hypo- 

 blast in front of the anus ; and so on. 



III. Classification. — After giving a brief history of our systematic 



