483 



growth of the stomach, known as the pyloric caecum, that has been de- 

 scribed for so many Styelids and Botryllids, will ultimately be found to 

 be of general occurrence in these two groups. With one exception, I 

 have found it in every species that I have been able to examine. In 

 many cases, I was at first unable to find it, but it was revealed by further 

 search. As yet I have not been able to find it in Heller's Poly carpa 

 elata. The poor state of preservation of the single specimen at my dis- 

 posal may be responsible for this. 



The pyloric caecum appears to be particularly distinct in small 

 individuals. Not only in species in which mature individuals are of 

 small size (e. g. Botryllidae and Polyzoinae), but also in small, immature 

 individuals of large species. In a specimen of Styela gibbsii (Stimps.) 

 V 2 mm long the pyloric caecum is very distinct, being as long as the 

 stomach itself. In the adult of this species it is scarcely distinguishable. 

 It is probable that in species in which it is quite unrecognizable in the 

 adult, it may be characteristic of the immature stages. 



Atrial Tentacles. These structures are not found, as far as my 

 knowledge goes, in any other family. They appear to be peculiar to 

 Styelids. They occur in all the Styelids that I have examined. I have 

 not been able to find them in Botryllus, although I have examined an 

 individual of that genus in serial sections. The atrial tentacles are 

 usually of small size and hence difficult to detect. Owing to their pe- 

 culiar structure, which will be described subsequently, they are fre- 

 quently removed with the test in dissection and of course cannot then 

 be found on the wall of the atrial siphon. They were first noticed by 

 Herdmanand described in the results of the Challenger Expedition. 

 Since that time they have been frequently mentioned in descriptions of 

 species, particularly in the last few years. They have not yet been found 

 in the majority of the species , but further search will doubtless show 

 that they are present in all. 



Their structure does not appear to have been the object of study. They 

 show a very curious condition, differing much from that of the oral ten- 

 tacles. In most Ascidians the junction of the test-bearing and testless 

 epithelia of the walls of the siphons is not very prominent. The epithe- 

 lial cells are usually somewhat higher at that point than elsewhere and 

 may project slightly into the siphon forming a ridge which runs circu- 

 larly around the siphon. This is the condition in Corella, Ckelyosoma 

 and Ascidiopsis. In the Ptychobranchiates this ridge becomes very 

 prominent, projecting so far as to form a distinct shelf or velum. The 

 velum is only slightly developed in Botryllus. In the Caesirids that I 

 have examined, it is quite narrow. In many Styelids and Tethyids it 

 is very broad and usually in these families it does not project into the 



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