HYDROIDA. BALE. V 



All the above-named species (with many others) agree in the 

 arrangement of the stem-internodes. Evei'y such internode gives 

 origin at the base to a pinna, which has a single unpaired 

 hydrotheca in the axil. Higher up, it bears the regular pair of 

 hydrotheca?, only it will be observed that in most cases the one 

 on the same side as the pinna is higher than the corresponding 

 one on the opposite side, being as it were crowded out of its 

 normal position owing to the presence below it of the pinna with 

 its axillary hydrotheca. In attenuated individuals of some of the 

 species, where the internodes are much elongated, the difference 

 of level may be so great that the paired condition is not at first 

 noticeable. 



The mode of ramification just described contrasts strongly 

 with that of the Dynamena-group, and the existence of an 

 entirely different arrangement of the hydrotheca? on the pinna?, 

 as compared with those on the stem, marks a distinct advance in 

 organisation. 



This arrangement of the stem-internodes, (which is especially 

 typical of the genus Sertularia), is not confined to the group 

 under consideration, but is found in a number of small delicate 

 forms such as S. tenuis, 8. inflata, and 8. biscuspidata. They 

 differ from the 8. elongata group not only in their more delicate 

 texture and small size, but in the fact that their pinna? are 

 wholly divided into internodes of equal value, and exhibit no 

 tendency towards the production of internodes of the second or 

 third order. They are thus more closely allied to the Dynamena- 

 group, and indeed are typical Dynamence as understood by 

 Kirchenpauer. The distinction between them and Schneider's 

 Dynamena-group may be well illustrated by the comparison 

 between 8. gracilis and 8. tenuis. Both these species are very 

 similar in the form of the hydrotheca? as well as the gonangia, 

 and both may exist in the unbranched condition. But when 

 S. gracilis produces side branches the arrangement of the 

 hydrotheca? on the stem is unaltered, and the stem and branches 

 are exactly alike. 8. tenuis on the other hand, when producing 

 side branches, invariably has them in the form of alternate 

 pinna?, with the complete differentiation of stem and pinna?, 

 as in the 8. elongata group, 



The peculiar characteristic of 8. elongata and its allies consists 

 in the general tendency for the older internodes of the pinna? 

 (or some of them) to bear two or three or even more pairs of 

 hydrotheca?, while on the distal portions one pair is the rule. 

 The prevalence of longer or shoi'ter internodes in particular 

 specimens examined has led to untenable distinctions being drawn, 

 such as the reference of some members of the group to Thuiaria 

 and others to Sertularia. 8. elongata, the most abundant species, 

 may serve to illustrate the habit of the group. In the smallest 



