Ctenophora

Ctenophores, commonly known as comb jellies, are a group of marine invertebrates belonging to the phylum Ctenophora. They are primarily planktonic organisms found in oceans worldwide, from surface waters to the deep sea. Known for their iridescent beauty and bioluminescence, ctenophores are characterized by cilia, called comb rows, which they use for locomotion.

1.0Structure Of Ctenophores

• The spherical body can be divided into two hemispheres. 
• The mouth lies at one end, or oral pole, and a sense organ is at the opposite end, or aboral pole.

Combplates

• Eight equally spaced rows of paddle plates are arranged on the sides of the body and are used in swimming. 
• The comb rows comprise a series of short ciliary plates or ctenes. The cilia are strong and propel the animal slowly through the water. 

Tentacles

• Two in number were found nearer to the aboral end on opposite body sides. They are extremely long, solid and retractile. 
• Tentacles emerge from deep ciliated epidermal blind pouches or tentacular sheaths. The tentacle bears short lateral branches or pinnae. 
• Nematocysts are absent, but tentacles possess peculiar adhesive cells called lasso cells or colloblasts that help capture Food.

2.0Sense Organs of Ctenophores

• The apical sensory organ is a deep-seated statocyst at the aboral pole. 
• It is lined by tall, ciliated epithelial cells. Statocyst contain statolith and balancers. 
• It is covered by a roof like a dome or bell, formed of fused cilia. 
• The sensory organ serves as an organ of equilibrium.

3.0Body Wall of Ctenophores

• They comprise an outer epidermis and an inner gastrodermis separated by a thick gelatinous mesogloea. 
• The epidermis is syncytial and contains many gland cells, sensory cells and pigment granules. 
• Mesogloea contains amoebocytes, connective tissue fibers, muscle fibers and some nerve cells. 

4.0Digestive System of Ctenophores

• Mouth slit-like situated in the center of the lower end. It leads into a long tubular pharynx lined with epidermis. 
• The pharynx opens into a small but wide stomach. It produces a system of five gastrovascular canals that extend throughout the jelly in a definite arrangement. 
• The stomach and gastrovascular canals are lined with gastrodermis. 
• Two anal canals open to the outside near the aboral sense organ, each by an anal pore.

• Colloblast Ctenophores feed on small planktonic organisms and are voracious. Food is captured by trapping in colloblast. 
• Digestion is extracellular in the pharynx and intracellular in gastrovascular canals. 

5.0Respiratory System of Ctenophores

• There are no respiratory structures. The gaseous exchange takes place through the general body surface. 

6.0Excretory System

• There are no definite excretory organs. 
• Cell rosettes consist of a double circlet of ciliated gastrodermal cells, surrounding openings leading from the gastrovascular canals to the mesogloea. 
• They may be excretory or osmoregulatory.

7.0Nervous System of Ctenophores

• There is no localized control centre. The epidermal nerve plexus is concentrated in a ring around the mouth and at the base of the comb rows, where it forms the radial nerves. 
• The nerves are not true nerves but the condensation of the nerve net. 
• The nervous system controls muscular movements and determines cilia's activity on the comb rows. 
• The aboral sense organ is a statocyst or balance organ that is useful in maintaining normal orientation. 

8.0Reproductive System and Development of Ctenophores

• All are hermaphrodites. Reproduction is sexual only, and asexual reproduction is absent. 
• Gonads develop from endoderm in the form of bands in the meridional canals of the gastrovascular system.

Development 

• Generally, fertilization is external. Cleavage is total, determinate, and unique in ctenophores and is called disymmetrical. 
• Usually, free swimming characteristic cydippid larva occurs, which undergoes a gradual metamorphosis. 
• Some ctenophores exhibit a strange phenomenon called misogyny, in which both the larva and adult reproduce sexually. 
• There is no alternation of generation. 
• Ctenophores have great powers of regeneration. 
• Lost or wounded parts, even the statocyst, are replaced or repaired by regeneration.