ACINETARIA. 827 



the apex of each point, to traverse the arm and enter the body. The arms 

 are slowly retracted when the animal assumes a temporary motile state. 

 No trace of the retracted arms is discernible, and when they reappear, 

 they do so as a process ending in one or more tubular points. The 

 Ophryodendridae are characterised by having one or more contractile 

 proboscides, simple in Acinetopsis, with the free end in Ophryodendron 

 beset with numerous flexible and pointed cirri 1 . 



The nucleus is single : in Hemiophrya it is at first horseshoe-shaped, 

 but is subsequently much branched, and resolved into thicker portions 

 connected by slender filaments ; and in Dendrocometes it branches and 

 extends unbroken through the colony and creeping stolon. It has a 

 nuclear membrane with dense contents : the chromatin is disposed in 

 fibrils when the nucleus divides. Paranuclei have been observed in a few, 

 e.g. Podophrya limbata, Acineta fetida. There is always one contractile 

 vacuole and sometimes more. A special vacuolar duct is found in Podo- 

 phrya Steinii y P. Wrzesniowskii and Dendrocometes. The contraction of 

 the vacuoles appears to be slow in some instances. Want of oxygen causes 

 them to become much distended. The presence of a large bubble of gas 

 (Carbon dioxide ?) and its gradual absorption has been recorded in a 

 Sphaerophrya (Engelmann). Trichocysts are said to occur in two species 

 of the genus Ophryodendron. 



Reproduction takes place by fission or gemmation, external or 

 internal. Fission is universal in the genus Sphaerophrya ; it has been 

 observed in Urmda, in Podophrya libera, P. fixa, P. mollis and Acineta 

 mystacina 2 . One of the parts is set free and retracts its tentacles com- 

 pletely except in Sphaerophrya, and becomes ciliate. The fission-product 

 in Urnula and Acineta mystacina is decidedly smaller than its parent. 

 External gemmation is characteristic of the genera Hemiophrya and 

 Ophryodendron. In the former the buds are produced on the anterior 

 face of the body, and so far as is known are ciliated on one aspect only. 

 They have been observed with tentacles developed before detachment. 

 As to the latter, the bud is non-ciliate, elongate, with a very mobile tubular 

 process or neck ; it is often termed ' lageniform zooid.' Internal gemma- 

 tion is very general, and it occurs in Ophryodendron side by side with 

 external. The process begins in Podophrya quadripartita by the forma- 



1 Maupas, op. cit. p. 328, compares the arms of Dendeocometes to a bundle of united tentacles. 

 On p. 356 he states that Koch has found that ' the proboscis of Ophryodendron has an identical 

 structure.' Koch's work, 'Zwei Acineten auf Plumularia setacea (Ellis),' Jena, 1876, has not been 

 accessible to me. 



2 Saville Kent describes the process in P. mollis as taking place thus : the animal fixes itself 

 by means of two tentacles in addition to its own peduncle ; the two tentacles fuse and become the 

 peduncle of the new animal which is formed by the division of the doubly attached body. He 

 remarks that unless witnessed from the beginning the process would be interpreted as one of con- 

 jugation. See 'Manual of Infusoria,' p. 821. Gruber states (Z. W. Z. xxxvi. p. 118) that he has 

 seen gemmation, fission, and the detachment of small fission-products in Acineta mystacina. 



