Though not obvious, cephalization does exist in an earthworm. The nervous system of the earthworm is distributed through the segmented body, along a nerve core, which lends support to the claim that earthworms have no cephalization; however, one particular part of this nervous system, an enlarged ganglion, does act as a simple brain, and it is located in the anterior portion of the earthworm’s anatomy. Hence, the earthworm exhibits cephalization.
According to Biology Online, ganglia are masses of nerve tissue or groups of nerve cell bodies, especially nerve cells external to the brain or spinal cord. In an earthworm, the ganglia essentially operate as the brain; however, it must be noted that these ganglia are distributed throughout the body of the earthworm. The singular of ganglia is ganglion.
Earthworm’s Basic Biology
An earthworm is an annelid: a type of invertebrate whose body is segmented. The body of an earthworm is divided internally and externally into well-defined, segments which are, except for the head and tail segments, essentially alike. These segments may be separated from each other by membranous partitions. The technical term for the type of segmentation witnessed in earthworms is “metamerism,” to differentiate it from other types of segmentation.
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Cephalization is the tendency in the development of animals for the important organs to become located in or near the head. This tendency is most pronounced in mammals such as humans, where the brain and other essential organs are located in the anterior (or upper) portion of the body; the tendency is least pronounced in invertebrates such as sponges which have neither a centralized nervous system nor coordinated behavior over the whole organism. According to an “Online Introduction to the Biology of Animals and Plants,” “segmented worms show cephalization. An earthworm is a segmented worm.”
Increasing cephalization refers to increasing complexity of the nervous system and localization in the anterior end of an organism. Sponges, which have no nervous system, are at one end of the spectrum while cnidaria, which have no ganglia, are at the other.
Earthworm’s Nervous System
The nervous system of the earthworm consists of an anterior, dorsal, ganglionic mass, or a brain, and a long ventral solid nerve cord with ganglionic swellings and lateral nerves in each segment. With earthworms, you have a nerve cord that runs the length of the body with ganglia at every segment, but also an enlarged cerebral ganglion located at the anterior end of the body. This single, enlarged ganglion acts as a simple brain, controlling and coordinating certain functions in the body.
Since cephalization is the concentration of nerve cells at one end of the body, and since the earthworm’s nervous system is distributed throughout its segments along the central nerve cord, it can be said that the earthworm lacks obvious cephalization; however, given that the englarged ganglionic mass is located in the anterior segment of the earthworm, and that this ganglionic mass operates as a simple brain, it cannot be denied that the earthworm exhibits some small degree of cephalization.