332 Mr. E, Kay Lankester on the 



of the mesoblast, between and around which the movement of 

 a fluid, so-called lymph, is possible. 



A blood-lymph-system or series of channels appears in its 

 simplest form in the flat-worms, where the main portion of 

 those channellings in the mesoblast, sometimes spoken of as 

 " water- vascular system," must be regarded as the commencing 

 differentiation of the blood-lymph vascular system. The true 

 nature of these channels is well seen in a transverse section, 

 such as that of Bothriocephalas given by Landois (Zeitschr. 

 f. Zool. 1872), or such as that of the Planarian Bipalium to be 

 described by my friend Mr. Moseley, who assigns to them the 

 same importance as is done here. The channels of the water- 

 vascular system in these cases are seen in section to be inter- 

 sected by long branching cells ; they are, in fact, only partial 

 excavations of the mesoblastic tissue. Such excavation, can-ied 

 to a greater extent and widened out, ultimately forms the 

 "perivisceral space" seen in many Nemerteans, and in all the 

 Gephyrea, Ch^etopoda, Echinodermata. When parts of this 

 excavation remain shut off from parallel parts, and either com- 

 municate or do not communicate with the larger sinus-like 

 spaces, the conditions are given for the further modification of 

 this primitive vascular channelling into distinct blood-vessels, 

 lacunae, and pericardial sinus-system, as in Mollusks, or into 

 a closed vascular system lying within a perivisceral sinus, as 

 in Chajtopoda, or (no perivisceral sinus being apparent) into 

 closed vessels containing haemoglobin surrounding organs, as 

 in some leeches, or, lastly, into great sinus-spaces opening 

 through a "lymph-system" into a closed system of blood- 

 vessels, as in Vertebrates. 



The orifices of the water-vascular system of the Planarians, 

 Cestodes, and Trematodes are, no doubt with reason, looked upon 

 as representing exactly the orifices of the "segment-organs" 

 of the Chsetopoda ; but we have no warrant for assuming that 

 more than the aperture and a first portion of the "canal " in 

 the flat- worms corresponds with the little trumpet-mouthed 

 tube which hangs freely in the large perivisceral space of a 

 Chsetopod, or such a leech as Branchiobdella. The observed 

 facts of development are not conclusive as they at present stand 

 as to the origin of the segmental organ of Chsetopoda. Kowa- 

 lewsky derives them from the middle layer in the case of 

 Euaxes ; but in view of the difficulties of the observation, and 

 of adverse considerations furnished by the facts of development 

 of apparently homogenous parts in Mollusks and Insects, an 

 argument caimot be based upon their mode of development ; 

 nor do the facts of development at present established lend 

 themselves to the decision of the question whether the flat- 



