OBSERVATIONS ON POLYZOA. 219 



of the oesophagus, the (Esophagal Retractors ; and one to 

 the region of the stomach, the Gastric Retractors. They 

 are active from the first, and appear to drag the polypide 

 inwards, stretching the endocyst of the parent, which is 

 joined to the loops, into a tube. This tube is the future 

 evaginable endocyst of the polypide ; and, as the various 

 organs are developing, it is everted little by little, becom- 

 ing gradually capable of the adult evagination. 



The tentacles of Cristatella ophidioidea are not fully 

 grown, nor the arms divaricated, until long after the evagi- 

 nation of the polypide is completed (PL 13, fig. 3, Y). 

 At this period the tentacles of the external rows near the 

 mouth are the longest, decreasing regularly to the mere 

 tubercles on the ends of the arms, and the internal tenta- 

 cles are not separated from each other, exhibiting only two 

 closely appressed lines of tubercles all of about equal length. 

 The division of the arms begins internally, and its progress 

 outwards may be followed by the gradually increasing 

 length of these interior rows, which retain their tubercular 

 character until this division commences (PL 9, fig. 14). 



The mode of reproduction by true ova, although detect- 

 ed by Dumortier and Van Beneden, was first fully describ- 

 ed by Prof. Allman. They are produced from the gemma 

 dot, a bud-like mass on the upper side of the endocyst in 

 the neighborhood of the orifice, which, during the fall, 

 when not filled with ova, becomes opaque and granular.* 



The testicle, first described by Dumortier and Van Bene- 

 den, arises from the funiculus, resembling in its mode of 

 formation, according to Prof. Allman, a true bud. The 



*Prof. Allman thus describes the earlier periods of the development 

 of the ovum. Monograph Fresh-water Polyzoa p. 33. 



"Development of the Ovum. — I have succecled in tracing the develop- 

 ment of the ovum through most of its stages in Alcyouella fungosa. 



In this polyzoon the mature ovum consists of a granular vitellus, 

 surrounded by a very evident vitellary membrane, on whose internal 

 surface the contents appear frequently to be aggregated in a coarse 

 granular layer. It presents a large germinal vesicle, and a very dis- 

 tinct germinal spot. After a time the germinal vesicle and the germi- 

 nal spot disappear, and the vitellus undergoes segmentation, and after 

 the mulberry-like condition thus induced has in its turn vanished, we 

 find the contents of the egg have assumed the form of a roundish or 

 oval body, richly ciliated on its surface, and provided with a large ceu- 



