ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 511 



developmental history of most Annelids, that of the Gephyrean under 

 discussion appears to be very short ; examples of shortened develop- 

 ment are, however, among Annelids to be observed in numerous 

 Oligochaeta and in the Hirudinea, though it is not of the same 

 character as in these. At the same time it may be supposed to be a 

 lately acquired arrangement. 



Compared with the well-known trochophore-larva, that of Sipun- 

 culus corresponds to a stage much further developed ; while there are 

 numerous points of resemblance, we find that, in the latter, the post- 

 oral circlet of cilia is not feebly but well developed ; further, the 

 trunk and the secondary ccelom are well developed ; provisional organs 

 appear to be absent, and the permanent retractors of the fore-body 

 and the renal organs of the trunk are in full activity. In fact, it 

 would seem that the trochophore stage is, in Sipunculus, passed 

 through during the much shortened embryonic life, and that the pro- 

 visional organs which characterize the head in that stage are com- 

 pletely suppressed. To what stage in the development of an Annelid 

 does, then, the young Sipunculus-larya correspond ? It can only be 

 compared to some much more developed condition from which, how- 

 ever, it will be found to differ in important particulars, and notably 

 in the absence of any indications of metamerism. 



Points of interest are also to be observed in the history of the 

 development of the muscular and nervous systems : compared with the 

 Annelid we find that the layer of circular muscles is developed much 

 earlier than that of the longitudinal, and that the latter never form 

 a continuous layer. The very early development of the two pairs of 

 retractors is also an important point, and may perhaps be regarded 

 as an instance of precocious differentiation, such as is often observed 

 with those organs which play a specially important part in the 

 organization of any class of animals. The rudiment of the ventral 

 medulla is median and completely unpaired, instead of, as in the 

 Annelids, presenting two lateral cords ; to this character, however, 

 Hatschek does not attach much importance, nor does he think that it 

 affects the homology of the structure in the two groups. 



A special comparison is next made between the Sipunculida and 

 the Echiurida, but his later observations have done nothing to shake 

 Hatschek's view that there is a gulf between the two groups, which 

 are so commonly united together ; the latter, indeed, form a sub- 

 class of the Annelida, while the " Sipunculacea " are to be regarded 

 as occupying the position lately held by the " Gephyrea." 



In a note on the head-cavities the author states that in Poly- 

 gordius the ccelom of the head appears to be formed by a secondary 

 outgrowth of processes from the ccslomic sacs ; in other words, it is 

 not, as some have thought, a direct continuation of the coelom. In 

 another note attention is directed to the late development of the 

 oesophageal commissure, which Hatschek, contrary to the opinion of 

 Kleinenberg and Balfour, believes to be a cenogenetic phenomenon. 



The author has made some speculations on the relation of the 

 axis of the trochophore to that of the gastrula, and finds that the 

 aboral end of the gastrula is directly converted into the apical area of 



