46 Coelenterata. 



piece becomes spherical ; from it grows out a tubular structure on which are 

 formed tentacles, mouth, etc. There are 3 types of origin of mesenteries 

 in laceration-pieces (1) bilateral, with 3 primary, complete, and similarly 



orientated pairs, (2) bilateral, with only 2 similar pairs, (3) biradial, with 

 2 growth zones. In laceration-pieces of S. there are various types but the bi- 

 lateral type, especially (2), is the most usual. In M. the 2nd and 3rd types, 

 especially the latter, are met with, only 2 cases of (1) were observed. In 

 Aiptasia the 3rd type was found with one exception. In laceration-pieces and 

 in pieces of the pedal disc, the mesenteries in the new growth zone do not 

 arise simultaneously. In the first type the differentiation proceeds from within 

 outwards, and the order of formation is that seen in actinian larvse. In the 

 2nd type the 1st pair arises as before, then the directives, both similarly orien- 

 tated, and finally a pair on the inner side of the 1st. In the biradial type 

 each new growth zone develops its mesenteries in a similar manner, but in 

 the case in which the second zone has no directives, its mesenterial anlagen 

 are rather later. The 2nd cycle arises as in ontogeny. 



Chi Id f 1 ) states that the wound reactions and general course of restitution 

 in Harenactis do not differ widely from those processes in Cerianthus. The 

 contraction following the wound is not purely muscular; closure of the wound 

 by new tissue is sometimes impossible, even after contraction has approximated 

 the edges. Growth of new tissiie occurs only under a certain degree of mecha- 

 nical tension, therefore none occurs on a single free cut surface. Closure by 

 new tissue occurs only when the parts are closely approximated to each other. 

 The conditions of formation of the thin membrane of new tissue indicate that 

 capillarity here plays an important role. Union may occur between any two 

 cut surfaces which come into contact, without any relation to the normal form, 

 e.g., in pieces from the stomodseal region, the body wall and stomodseum unite 

 so that the stomodseum is open to the exterior at both ends. As in Cer. the 

 tentacles are formed as localised differentiations in a continuous sheet of tissue. 

 The rapidity of wound closure and of oral restitution decreases with increasing 

 distance from the oral end ; the decrease is small in the oral two-thirds of the 

 body but much greater in the aboral third. In small pieces from the extreme 

 aboral end, oral restitution does not occur, but closure of the wound may take 

 place slowly. When oral restitution does occur in pieces from the proximal 

 region the number of tentacles is sometimes 12 instead of 24, probably because 

 of the disappearance of the 2nd cycle of mesenteries, a phenomenon of size 

 as well as of region. 



Child ( 2 ) finds that in the stomodseal region of Harenactis, tentacles and disc 

 are always formed at both ends of pieces, provided they become sufficiently 

 distended. The aboral mouth is the result of the fusion of the cut end of the 

 body wall with the cut end of the stomodseum. Complete or partial oral 

 discs develop according as the operation involves the whole cross-section of 

 the body or only a part. The formation of aboral is less rapid than that of 

 oral tentacles in the same piece and also than that of oral tentacles from the 

 same level of the body. When only the most proximal regions of the body 

 are removed restitution of a foot occurs, but as the portion removed becomes 

 larger the restitution becomes less rapid and complete until, when most of the 

 region below the stomodseum is removed, only slight restitution occurs. The 

 incompleteness of aboral restitution in these cases depends on the environmental 

 coriditions ; in normal specimens kept in water without sand, so that they cannot 

 burrow, considerable reduction and atrophy occur, mostly in the proximal 

 region. - The regulatory phenomena of polarity fall under 4 heads 



