On the Structure and Affinities of Mnestra parasites Krohn etc. 39 



In some individuals , a cnidact may be longitudinally divided 

 into two parts (tig. 1 radius 2), tlius forecastìng the condition des- 

 scribed by Mayer in the new species from Florida, Ectopleura minerva^ 

 in which two cnidacts proceed from each tentacle-base to the aboral 

 pole. In Mnestra the streaks are broad near the tentacles, narrow 

 near the aboral pole; a very Constant feature seems to be a distinet 

 broadening- near the middle part of their course. 



The nematocysts are in a high state of functional activity 

 (tig. 8) and must be very eifective as weapons of ofifeuce or defence. 

 A theory to account for their reteution in the scssile medusa will 

 be considered below. 



In the possession of a ring of cuidoblasts round the margin of 

 the umbrella, Mnestra is possessed of an organ rare amongst the 

 Leptolinae, and it may well be considered as the first appearance 

 of such a nettle ring or peronium as is to be found among the Grery- 

 onidae or Trachynemidae. In Mnestra however, ali the nematocysts 

 are in a state of oÖensive or defensive activity, whereas amongst so 

 many of the Trachyliuae, they are degraded into parts of a mere 

 skeletal conglomerate. A similar marginai development of cuido- 

 blasts has been recorded in Gemmaria implexa and in Clavatella 

 (= Eleutheria] both Cladonemidae. 



The Tentacles. 



The tentacles should be four in numbei-, one at the end of 

 each radiai canal; but individuals with four in a state of complete 

 development are very rare. One, two, three or even ali four may 

 be reduced. The majority of specimens had two adjacent tentacles 

 reduced to mere knobs and many had three or even four reduced 

 to this condition (tigs. 36 — 40). It seemed to me that in some cases 

 the reduction of the tentacles to knobs was a mark of old age. 



A well developed tentacle of Mnestra is a compound hollow 

 tentaele of the semipinnate type characteristic of the Cladonemidae 

 (figs. 1, 14). The base is swollen and bulb-like; the distai portion 

 is slender, tapering and very extensile, hearing a row of small, 

 stalked club-shai)ed bodies along the enti re length of the aboral 

 side. These bodies (v. infra p. 4o and tigs. 16, 17) contain cnido- 

 blasts and are undoubtedly homologous with the similar and simi- 

 larly situated organs of several Cladonemid genera. For these 

 Organs the terms "Nebenfäden" and "tentaculae" bave already been 



