638 MR. E. II. LANKESTER ON SOME LOWER ANNELIDS. 



may be one or two clustered masses of granules ; these may be broken up into smaller 

 clusters enclosed by a sort of cell-wall ; or the granules may be free and separate ; or 

 they may have assumed the form of hyaline corpuscles with nucleus and nucleolus 



(fig. 2G «, c, b, d). 



Segment-Organs.— -These organs have not hitherto been mentioned by those who have 



incidentally alluded to other points in the anatomy of Chcstogaster. They vary very 

 much in their development in different individuals. In some they are only to be de- 

 tected, by most careful adjustment, as minute tortuous tubes lying beneath the integu- 



In others they have attained a considerable size, and hang as folded masses in 

 the perivisceral cavity. In no case are they large enough to admit of their true struc- 

 ture or parts being clearly recognized. Figure 29 represents all I have been able 

 to see in the asexual Chcutogaster* . I could not detect the free end, corresponding to 

 the funnel or " entonnoir vibratile" in other worms; but at one end there appeared to 

 be a dilatation representing the atrium. A pair exists in front of the first pair of 

 abdominal bristle-bundles, and another pair between each succeeding pair of bristle- 

 bundles. "Whether they open externally or are ciliated I cannot say ; but it cannot be 

 doubted that they represent the segment-organs of other Annelids. 



The Number of Segments forming cm Individual. — I have already mentioned that this 

 is a difficult matter to decide, inasmuch as zooids are continually being produced poste- 

 riorly, and that not in a linear progression, but by a continuous growth posterior to the 

 third pair of abdominal bristle-bundles. Do the cephalic pair of bristle-bundles and the 

 three abdominal pairs each indicate a segment or, rather, a head-segment and three 

 ordinary segments ? I think there is reason to answer this in the affirmative, since a 

 sort of septal muscle or diaphragm divides off the pharyngeal portion from that in which 

 the intestine commences as a stomach. This, again, is marked off by an obscure dia- 

 phragm from the succeeding region, which is also thus obscurely divided into segments 

 corresponding to the bristle-bundles. In each of the divisions thus made, there is, 

 exeepting in the pharyngeal, a pair of segment-organs ; but there is no constriction of the 

 anterior part of the worm in correspondence with these supposed segments. 



Segmentation in Vermes may be regarded as (and probably is) the result of the arrest 

 of the production of zooids by terminal budding. If this be the case, the unparalleled 



paucity of segments exhibited by Chcstogaster is directly connected with its enormous 

 fertility in the production of zooids. The " tendency to integration" (Herbert Spencer)\ 



Of 



Mr. Herbert Spencer has himself applied 



Evolution to the explanation of what has been vaguely termed « vegetative repetition/' In his excellent chapter 



Biology 



ondary aggrc 



There 



together a most convincing and interesting series of facts to support his argument 



does not touch, which I think is elucidated by the study of the growth of zooids in such Annulosa as Chattogaster 

 and that is the morphology of the head. Is the head, in those Annulosa in which it presents a quasi-segmented con 

 dition, to be regarded as composed of modified zooids? or is its segmentation (in the higher Annelids and in Arthro 

 poda, o be considered ^W! In many mollusca and in the vertebrata we find a head presenting a seg 

 mented condition or a certain repetition of parts. In these the structure is regarded as superinduced in a secondar 



