Naviculacea.'] infusorial animalcules. 301 



covered, each with a smooth, cylindrical membrauc — the young 

 sporangia, which gradually increase in length (P. 14, f. 5, 6) retain- 

 ing nearly a cyKndrical form (P. 14, f. 7) until they far exceed in 

 dimension the parent frustules, and, at length, when mature, become, 

 like them, transversally striated upon the surface (P. 14, f. 8.) 

 Around the whole structure a considerable quantity of mucus has, 

 during this time, been developed, by which the empty frustules are 

 held attached to the sporangia (P. 14, f. 5 to 8.) 



In the immature condition, it happens that the sporangia, in many 

 species, resemble in general characters the mature frustules of 

 another species, or even of an allied genus. Thus the sporangia of 

 Gomphonema minutissimum, and of G. dichotomum, have a close re- 

 semblance to frustules of Cocconema. On tjie other hand, in some 

 genera, as in Cocconema, the sporangia take on at once the exact 

 characters of the ordinary fi'ustules, from which they differ only in 

 their exceeding that of the majority of the latter in dimensions. 



When a sporangium in a ti'ansitional condition is like the frustule 

 of another genus, we are assisted in. distinguishing its true nature 

 and affinity, oftentimes by the persistence of the mucus diffused 

 around it ; or, by continued observation, we may witness its 

 assumption ultimately of its true specific characters, including the 

 development of its pedicle or stalk, where the possession of such an 

 organ is a characteristic (as in Gomphonema.) The development of 

 the sporangium being complete, it enters on an independent existence, 

 which it displays by undergoing fissiparous division, as in common 

 frustules. 



It is veiy probable that transitional forms have been described as 

 particular species, or located in wrong genera. Thus, Mr. Thwaites 

 thinks that Kiitzing's £pithemia Vertagas is the sporangium of 

 Eunotia targkla. 



In different genera, slight variations are met with in the method 

 of conjugation : thus, in some species of Gomphoyiema the sporangia 

 lie in a du'cction parallel to the empty frustules, instead of across 

 them, as described in Eunotia turgida. Again, there are examples (in 

 Gomphonema minutissimum and Fragilaria pectinalis), where, instead of 

 the conjugated frustules separating into two halves, only a slit 

 appears at one end, to serve for the escape of the endochrome. 



